Engine Controls - Basic Diagnostic Procedures - Gasoline Chevrolet Suburban IX

MODEL IDENTIFICATION

MODEL IDENTIFICATION

INTRODUCTION

The following diagnostic steps help prevent overlooking simple problems. This is also where to begin diagnosis for no-start condition.

The first step in diagnosing any driveability problem is verifying the customer's complaint by test driving vehicle under the conditions in which the problem reportedly occurred.

Before entering self-diagnostics, perform a careful and complete visual inspection. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. Before condemning the computerized system, perform each test listed.

Note. Unless otherwise instructed in test procedures, perform all voltage tests using a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance.

The following table provides location of commonly used diagnostic information. These former "A" and "C" charts are now written in text and inserted into appropriate location of engine performance workflow. To learn about engine performance workflow, see HOW TO USE THE ENGINE PERFORMANCE SECTION in this section.

GENERAL MOTORS A & C CHART REFERENCE

VISUAL INSPECTION

Visually inspect all electrical wiring. Look for chafed, stretched, cut or pinched wiring. Ensure electrical connectors fit tightly and are not corroded. Ensure vacuum hoses are properly routed and not pinched or cut. If necessary, see VACUUM DIAGRAMS article to verify routing and connections. Inspect air induction system for possible vacuum leaks.

Compression

Check engine mechanical condition using a compression gauge, vacuum gauge or engine analyzer capable of performing a relative compression test. If using engine analyzer, see engine analyzer instruction manual for availability and description of relative compression feature.

Exhaust System Backpressure

Before replacing any components, check exhaust system for restrictions. Use a vacuum gauge or a low-pressure (1-5 psi) gauge to check exhaust system.

If using a vacuum gauge, connect gauge to intake manifold vacuum, and start engine. Observe gauge while holding throttle steady at 2000-2500 RPM. If vacuum gauge reading slowly drops after stabilizing, exhaust system may be restricted.

1. Check At Oxygen Sensor Remove oxygen sensor. Install backpressure tester in place of oxygen sensor. After test is completed, coat oxygen sensor threads with anti-seize compound before installation.
2. Diagnosis Start engine and bring to operating temperature. Increase engine speed to 2000-2500 RPM. Note gauge. Exhaust system is restricted if reading exceeds 1.25 psi (.09 kg/cm 2 ). Check exhaust system for collapsed pipe, heat distress and possible internal muffler failure. If none of these conditions exists, check for restricted catalytic converter. Replace as required.

DEFINITION

No-start is defined as engine cranks okay but does not start. Engine may fire a few times.

Note. Before performing following tests, check battery condition, engine cranking speed and fuel supply.

General Inspection

1. Ensure proper starting procedure is being used. Visually check vacuum hoses for splits, kinks and proper connections, as shown on Vehicle Emission Control Information label. Check ignition wires for cracking, hardness and proper connections at both coil pack and spark plugs.
2. Remove spark plugs. Check and replace as necessary. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

Ignition System

1. Perform «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__on-board-diagnostic-obd-system-check). After performing OBD system check, go to next step.
2. Install Tech 1 scan tool. Using scan tool, monitor Throttle Position (TP) sensor voltage. Ensure is throttle closed. If TP sensor value is greater than 2.5 volts, diagnose using DTC P0123. See TESTS W/CODES article. If TP sensor voltage is less than 2.5 volts, go to next step.
3. Using scan tool, monitor Engine Coolant Temperature (ECT) sensor. If ECT sensor value is less than -30°C, diagnose using DTC P0118. See TESTS W/CODES article. If ECT sensor is greater than -30°C, go to next step.
4. Turn ignition off for 10 seconds. Turn ignition on. Fuel pump should operate for about 2 seconds. If fuel pump operates as specified, go to next step. If fuel pump does not operate as specified, go FUEL SYSTEM check.
5. Turn ignition off. Disconnect injector harness connector at intake manifold. Using a test light connected to ground, probe injector feed circuit (Pink wire). (Scheme 107) Ignition ON engine OFF. If test lamp illuminates flashes, go to next step. If test light does not illuminate, go to step 8).
6. Disconnect one spark plug wire. Install Spark Tester (ST-125). Crank engine. If spark is present, go to next step. If spark is not present, see C-4, IGNITION CONTROL CIRCUIT in SYSTEM/COMPONENT TESTS.
7. Reconnect spark plug wire. Turn ignition off. Connect fuel pressure gauge. Turn ignition on and observe fuel pressure. On 7.4L SFI engines, fuel pressure should be 55-61 psi (3.9-4.3 kg/cm 2 ). On all other engines, fuel pressure should be 60-66 psi (4.2-4.6 kg/cm 2 ). If fuel pressure is as specified, perform fuel injector balance test. See INJECTOR BALANCE TEST under FUEL SYSTEM in SYSTEM/COMPONENT TESTS article. If fuel pressure is not as specified, «BASIC FUEL SYSTEM CHECKS»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline).
8. Repair injector ignition feed circuit (Pink wire). (Scheme 107) After repairs, go to next step.
9. Using scan tool, select DTC, CLEAR INFO function. Attempt to start engine. If engine start and continues to run, go to next step. If engine does not start or starts and dies, go to step 2).
10. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN. If DTC(s) are displayed, diagnose DTC(s). See TESTS W/CODES article. If DTC(s) are not present, go to next step.
11. Select CAPTURE INFO, REVIEW INFO function. If additional DTC(s) are displayed, diagnose DTC(s). See TESTS W/CODES article.

Scheme 107

Fuel System

1. Perform «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__on-board-diagnostic-obd-system-check). After performing OBD system check, go to next step.
2. Turn ignition off for about 10 seconds. Turn ignition on. Fuel pump should operate for about 2 seconds. If fuel pump operates as specified, go to IGNITION SYSTEM check. If fuel pump does not operate as specified, go to next step.
3. Connect a fused jumper wire between battery positive and fuel pump test terminal. If fuel pump operates, go to next step. If fuel pump does not operate, go to step 12).
4. Disconnect jumper wire from test terminal. Turn ignition off for 10 seconds. Turn ignition on. Fuel pump should operate for about 2 seconds. If fuel pump operates as specified, go to step 25). If fuel pump does not operate as specified, go to next step.
5. Remove fuel pump relay. Turn ignition on with engine off. Using a test light connected to ground, probe fuel pump relay harness connector battery feed terminal (Orange wire). (Scheme 108) If test light illuminates, go to next step. If test light does not illuminate, go to step 14).
6. Connect test light between ground and fuel pump relay harness connector battery feed circuit. If test light illuminates, go to next step. If test light does not illuminate, go to step 15).
7. Connect test light between ground and fuel pump relay terminal. Turn ignition off for 10 seconds. Turn ignition on. If test light illuminates for 2 seconds and then turns off, go to next step. If test light does not illuminate, go to step 16).
8. Check for faulty fuel pump relay harness connector. Repair as necessary. After repairs, go to step 11). If connector is okay, go to next step.
9. Replace fuel pump. After repairs, go to step 25). If fuel pump is okay, go to next step.
10. Start and warm engine to normal operating temperature. Ensure oil pressure is within normal range. Disconnect fuel pump relay. If engine continues to run, go to next step. If engine stalls/dies, go to step 17).
11. Reinstall fuel pump relay. Turn ignition off. Using a test light connected to ground, probe fuel pump test connector. If test light illuminates, go to step 18). If test light does not illuminate, go to step 25).
12. Disconnect fuel pump harness connector. Using a fused jumper wire connected to battery voltage, jumper fuel pump test terminal. Using a test light connected to ground, probe fuel pump feed circuit. If test light illuminates, go to next step. If test light does not illuminate, go to step 20).
13. Connect test light to fuel pump ground terminal on harness connector. (Scheme 108) If test light illuminates, go to step 19). If test light does not illuminate, go to step 21).
14. Repair open in battery feed circuit to fuel pump relay harness connector terminal. After repairs, go to step 25).
15. Repair open in fuel pump relay ground circuit. After repairs, go to step 25).
16. Check for faulty connection at VCM fuel pump relay control circuit. Check for open or short to ground in harness between fuel pump relay control connector terminal and fuel pump relay control circuit. (Scheme 108) Repair as necessary. After repairs, go to step 22). If circuits are okay, go to step 24).
17. Check for faulty oil pressure switch connector terminals. Repair as necessary. After repairs, go to step 22). If connector terminals are okay, go to next step.
18. Replace oil pressure switch. After repairs, go to step 25).
19. Replace fuel pump. After repairs, go to step 25).
20. Repair open between fuel pump test terminal and fuel pump feed circuit at fuel pump harness connector. After repairs, go to step 25).
21. Repair open in fuel pump harness ground circuit. After repairs, go to step 25).
22. Repair affected circuit as necessary. After repairs, go to step 25).
23. Repair connectors as necessary. After repairs, go to step 25).
24. Replace VCM. Perform VCM relearn procedures. After repairs, go to next step.
25. Clear DTC(s) using scan tool. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and stalls/dies, go to step 2).
26. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN function. If any DTC(s) are present, diagnose DTC. See TESTS W/CODES article. If DTC(s) are not present, go to next step.
27. Select CAPTURE INFO, REVIEW INFO function. If additional DTC(s) are displayed, diagnose DTC(s). See TESTS W/CODES article.

Scheme 108

FUEL SYSTEM PRESSURE RELIEF

Fuel system is under pressure, which must be relieved before servicing fuel system. Fuel pressure may be relieved by one of the following methods.

1. Disconnect negative battery terminal. Loosen fuel filler cap.
2. Install Fuel Pressure Gauge (J-34730-1A) to fuel pressure test head. Wrap shop towel around pressure connection when installing fuel pressure gauge to absorb fuel leakage. Place gauge bleed hose in container. Open bleed valve to relieve fuel pressure.

Test Description

The numbers below refer to the step numbers in the diagnostic procedure.

1. 2 Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition leaving the engine OFF, the fuel pressure should be 60-66 psi (415-455 kPa) when the pump is running. This pressure is controlled by a spring pressure within the regulator assembly.
2. 4 The fuel pressure that continues to fall is caused by one of the following items: The in-tank modular fuel sender is not holding pressure. The fuel pressure regulator valve is leaking. A Central SFI injector is leaking. Fuel line is leaking.
3. 15 If the fuel pressure is less than 55 psi (379 kPa) at idle, it falls into the following 3 areas: The pressure is regulated but less than 55 psi (379 kPa). The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. See FUEL SYSTEM test in «A-3, NO-START DIAGNOSIS»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline) . A restricted flow causing pressure drop. Normally, a vehicle with a fuel pressure of less than 44 psi (300 kPa) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged fuel filter. A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.
4. 21 DO NOT allow the fuel pressure to exceed 75 psi (517 kPa). Excessive pressure may damage the fuel system. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the fuel pump commanded ON, the fuel pressure should rise above 66 psi (455 kPa), as the valve in the return line is partially closed.
5. 26 When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure. However, the pressure at idle should be less, indicating the pressure regulator control.
6. 29 If the injector(s) is not grounded, physically remove suspect injector(s) and inspect for a fuel leak while system is pressurized. If the fuel injector(s) are not leaking and fuel pressure is not holding, the fuel pressure regulator should be inspected for a slow diaphragm leak.

1. Perform «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__on-board-diagnostic-obd-system-check) . After performing OBD system check, go to next step.
2. Turn ignition off. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__fuel-system-pressure-relief) . Connect Fuel Pressure Gauge (J-34730-1A) to fuel pressure test port. Turn ignition on with engine off. Bleed air out of fuel pressure gauge into container. The ignition may have to be cycled off for 10 seconds, then back on more than once in order to obtain maximum fuel pressure. Monitor fuel pressure with fuel pump running. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm 2 ) on Central Sequential Fuel Injection (CSFI) fuel system and 56-62 psi (4.0-4.4 kg/cm 2 ) on Sequential Fuel Injection (SFI) fuel system, and should hold steady. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to step 8 .
3. Turn ignition off for 10 seconds with A/C off. Turn ignition on. Fuel pump should run for 2 seconds. Observe fuel pressure after pump stops running. Fuel pressure may vary a few pounds, then should hold steady. After fuel pump stops, fuel pressure should stabilize and remain constant at 55-60 psi (3.9-4.2 kg/cm 2 ) on CSI system and 48-52 psi (3.4-3.7 kg/cm 2 ) on SFI system. If pressure is within specification and holds, go to step 5 . If pressure is not as specified, go to next step.
4. Turn ignition off for 10 seconds with A/C off. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__fuel-system-pressure-relief) . Disconnect fuel filter. Drain any remaining fuel from fuel pipes into container. Install an appropriate fuel line shutoff valve in place of fuel filter. Open shutoff valve. Turn ignition on. Check for leaks at shutoff valve. Start engine and idle for 30 seconds. Turn ignition off for 10 seconds. Turn ignition on with engine off. Using scan tool, command fuel pump on. Wait for fuel pressure to build. Close shutoff valve. Fuel pressure should remain constant at 55-60 psi (3.9-4.2 kg/cm 2 ) on CSI system or 48-52 psi (3.4-3.7 kg/cm 2 ) on SFI system. If pressure is as specified and holds, go to step 12 . If pressure is not as specified or is as specified but does not hold, go to step 6 .
5. Start engine. Warm engine to normal operating temperature. From idle, open throttle quickly, while observing fuel pressure. The fuel pressure should approach the top of the specified range. If fuel pressure is not within specification, or if fuel pressure is within specification, but fuel pressure is suspected of decreasing during acceleration, cruise or hard cornering, go to step 27 . If fuel pressure is within specification and fuel pressure is not suspected of decreasing during acceleration, cruise or hard cornering, see «FUEL INJECTION TROUBLE SHOOTING»(/chevrolet/suburban/ix-1991-2001/remont/basic-trouble-shooting/#basic-trouble-shooting-general-information__fuel-injection-trouble-shooting) under ENGINE PERFORMANCE in TROUBLE SHOOTING - BASIC PROCEDURES article in GENERAL INFORMATION.
6. Inspect the fuel feed line between the CSI/SFI unit and the shut-off adapter. If fuel feed line is faulty, go to step 25 . If fuel feed line is okay, go to next step.
7. Open shutoff valve. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__fuel-system-pressure-relief) . Disconnect fuel return pipe union located near fuel filter. Drain remaining fuel from fuel pipes into container. Install appropriate fuel line shutoff valve in place of fuel return pipe union. Open shutoff valve. Using scan tool, command fuel pump on. Check for leaks at fuel shutoff valve fittings. Start engine and idle for 30 seconds. Turn ignition off for 10 seconds. Turn ignition on with engine off. Using scan tool, command fuel pump on. Wait for fuel pressure to build. Close shutoff valve installed on fuel return pipe. If fuel pressure remains constant, go to step 22 . If fuel pressure does not remain constant, go to step 29 .
8. Turn ignition on with engine off. Using scan tool, command fuel pump on. Fuel pump should run for 2 seconds, and then turn off. Observe fuel pressure after fuel pump stops running. If fuel pressure is present, go to step 13 . If fuel pressure is not present, go to next step.
9. Using scan tool, command fuel pump on. Listen for operation of fuel pump. If fuel pump operates, go to next step. If fuel pump does not operate, perform FUEL SYSTEM test in «A-3, NO-START DIAGNOSIS»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline) .
10. Check for the following conditions: No fuel in the fuel tank (do not rely on fuel gauge reading). Plugged in-line fuel filter. Plugged or damaged fuel pump strainer. Restricted fuel line. On dual tank models, check the auxiliary tank for fuel. If a problem is found, go to next step. If no problem is found, go to step 12 .
11. Repair or replace as necessary. After repairs, go to step 30 .
12. Check for a leaking fuel feed pipe in between the shut-off adapter and the modular fuel sender. Repair as necessary. If no leak is present, replace the fuel sending unit. See «FUEL PUMP»(/chevrolet/suburban/ix-1991-2001/remont/removal-installation/#engine-controls-removal-installation-overhaul-gasoline__fuel-pump) under FUEL SYSTEM in REMOVAL & INSTALLATION article. After repairs, go to step 30 .
13. Turn ignition on with engine off. Using scan tool, command fuel pump on. Fuel pump should run for 2 seconds. Observe fuel pressure. If fuel pressure is greater than 66 psi (4.6 kg/cm 2 ) on CSI system or greater than 62 psi (4.4 kg/cm 2 ) on SFI system, go to next step. If fuel pressure is not as specified, go to step 15 .
14. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__fuel-system-pressure-relief) . Disconnect engine compartment fuel return line. Attach a flexible hose to fuel return line. Place other end of hose in container. Using scan tool, command fuel pump on and observe fuel pressure within 2 seconds. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm 2 ) on CSI system and 56-62 psi (4.0-4.4 kg/cm 2 ) on SFI system. If fuel pressure is as specified, go to step 16 . If fuel pressure is not as specified, go to step 17 .
15. Turn ignition on with engine off. Using scan tool, command fuel pump on. Fuel pump should run for 2 seconds. Observe fuel pressure. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm 2 ) on CSI system and 56-62 psi (4.0-4.4 kg/cm 2 ) on SFI system. If fuel pressure is less than specified, go to step 19 . If fuel pressure is not less than specified, go to step 30 .
16. Locate and repair restriction in fuel return line. After repairs, go to step 30 . If fuel return line is not restricted, go to next step.
17. Check for restriction in engine fuel return outlet pipe at point where fuel line was disconnected. If fuel return pipe is restricted, go to next step. If fuel return pipe is not restricted, go to step 22 .
18. Repair/replace engine fuel pipes as necessary. After repairs, go to step 30 .
19. Inspect in-line fuel filter and fuel line for restriction. If restriction is found, go to next step. If restriction is not found, go to step 21 .
20. Repair/replace in-line fuel filter or fuel line as necessary. After repairs, go to step 30 .
21. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__fuel-system-pressure-relief) . Disconnect fuel return pipe union located near fuel filter. Drain remaining fuel from fuel pipes into container. Install appropriate fuel line shutoff valve in place of fuel return pipe union. Open shutoff valve. Using scan tool, command fuel pump on. Bleed air out of fuel pressure gauge into container. DO NOT allow fuel pressure to exceed 75 psi (4.5 kg/cm 2 ). Excess pressure may damage fuel pressure regulator. Slowly close shutoff valve installed on fuel return pipe. Fuel pressure should increase to greater than 66 psi (4.6 kg/cm 2 ) on CSI system or 62 psi (4.4 kg/cm 2 ) on SFI system. If fuel pressure increases as specified, go to next step. If fuel pressure does not increase as specified, go to step 23 .
22. Replace fuel pressure regulator. After repairs, go to step 30 .
23. Check for the following conditions: Low fuel pump voltage due to high resistance connections. High fuel pump voltage due to internal pump binding. Poor fuel pump ground. Partially restricted fuel pump strainer. Incorrect or faulty fuel pump. If any of the listed conditions are present, go to next step. If none of the listed conditions are present, go to step 12 .
24. Repair or replace as necessary. After repairs, go to step 30 .
25. Replace fuel feed line. After repairs, go to step 30 .
26. Start engine and allow it to reach normal operating temperature. Open throttle quickly. Observe fuel pressure. Fuel pressure should increase to near 66 psi (4.6 kg/cm 2 ) on CSI system or 62 psi (4.4 kg/cm 2 ) on SFI system. If fuel pressure increases quickly, see appropriate TROUBLE SHOOTING - NO CODES article. If fuel pressure does not increase quickly, go to next step.
27. Check for the following conditions: In-line fuel filter for restriction. Fuel feed pipe for restriction. Mispositioned or damaged fuel pump strainer. Fuel pump flex pipe for leaks. Vehicle is equipped with correct fuel pump. Fuel pump electrical wiring for high resistance. If any of the listed conditions are present, go to next step. If none of the listed conditions are present, go to step 12 .
28. Repair or replace as necessary. After repairs, go to step 30 .
29. Locate and replace any leaking injector(s). See FUEL INJECTOR BALANCE TEST under FUEL SYSTEM in appropriate SYSTEM & COMPONENT TESTING article. After repairs, go to next step.
30. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If additional DTCs are present, diagnose DTCs. See appropriate SELF-DIAGNOSTICS article. If DTCs are not present, system is okay.

INJECTOR CIRCUIT DIAGNOSIS (A-4)

Note. This test only applies to vehicles equipped with a VCM.

1. «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__on-board-diagnostic-obd-system-check) . After performing OBD system check, go to next step.
2. Disconnect injector connector at intake manifold. Turn ignition on. Using test light connected to ground, probe ignition feed circuits. See WIRING DIAGRAMS article. If test light illuminates, go to next step. If test light does not illuminate, go to step 7).
3. Install injector test light to each injector circuit, one at a time. Crank engine. If injector test light flashes on each injector, check fuel injector(s). See FUEL SYSTEM in SYSTEM/COMPONENT TESTS article. If injector test light did not flash on an injector circuit, go to next step.
4. If injector test light was on steady on an injector circuit, go to next step. If injector test light did not illuminate on an injector circuit, go to step 6).
5. Install injector test to affected injector. Turn ignition off. Disconnect Blue and Red VCM connectors. Turn ignition on, with engine off. If injector test light turns off, go to step 11). If injector test light stays on, go to step 8).
6. Check for open or short in battery circuit to affected injector. If circuit is open or shorted, go to next step. If circuit is okay, go to step 9).
7. Repair circuit as necessary. After repairs, go to step 12).
8. Repair short to ground in injector control circuit. After repairs, go to step 12).
9. Check terminal contact at VCM connector. If terminal contact is faulty, go to next step. If terminal contact is okay, go to step 11).
10. Repair terminal contact. After repairs, go to step 12).
11. Replace VCM. Reprogram replacement VCM. After replacing VCM, go to next step.
12. Using scan tool, select DTC, CLEAR INFO mode. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and stalls/dies, go to step 14).
13. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN mode. If other DTC(s) are present, diagnose affected DTC(s). If no other DTC(s) are present, go to next step.
14. Using scan tool, select CAPTURE INFO, REVIEW INFO. If additional undiagnosed DTC(s) are present, diagnose affected DTC(s). If DTC(s) are not present, system is okay.

CIRCUIT DESCRIPTION

System includes a distributor, ignition coil, ignition control module, secondary wires, spark plugs, VCM and crank sensor. Ignition system is controlled by Vehicle Control Module (VCM). The VCM monitors the information from various engine sensors, computes the desired spark timing and controls the dwell and firing of the ignition coil via an (IC) line to the ignition control module.

ENHANCED IGNITION SYSTEM CHECK

Note. Before clearing DTCs, use scan tool Capture Info to save Freeze Frame and Failure Records for reference. Control module's data is deleted once Clear Info function is used.

1. Perform Powertrain On-Board Diagnostic (OBD) System Check.
2. Check spark plug wires for open circuits, cracks or improper seating of terminals at spark plugs, distributor and ignition coil before proceeding with test. Check spark at plug with J-26792 spark tester or equivalent while cranking. (If there is no spark on one wire, check a second wire). A few sparks then nothing is considered no spark. If adequate spark is present, go to HARD START diagnosis. If adequate spark is not present, go to step 3).
3. Remove coil wire from distributor cap. Insert J-26792 spark tester into the coil wire and clamp the tester onto a ground. Crank engine. If adequate spark is present, go to step 13). If adequate spark is not present, go to step 4).
4. Measure coil wire resistance using a DVOM. The resistance should be approximately 1000 ohms/inch. If resistance is within specified value, go to step 5). If resistance is not within specified value, go to step 32).
5. Disconnect ignition coil harness connector. Probe ignition coil connector terminal C with a test lamp connected to battery positive terminal. Crank engine. If test lamp is flashing while cranking engine, go to step 7). If test lamp is not flashing while cranking engine, go to step 6).
6. Turn OFF ignition. Probe ignition coil harness connector terminal C with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to step 28). If not, go to step 9).
7. Probe ignition coil harness connector terminal B with a test lamp connected to battery positive. If test lamp is illuminated, go to step 29). If not, go to step 8).
8. Ignition ON, engine OFF. Measure terminal A voltage with a DVOM connected to a ground. If DVOM measures above 10V, go to step 21). If not, go to step 18).
9. Turn OFF ignition. Disconnect ignition control module harness connector. Check for an open circuit between ignition coil harness connector terminal C and ignition control module harness connector terminal D. If circuit is open, go to step 27). If circuit is not open, go to next step.
10. Disconnect ignition control module harness connector. Ignition ON, engine OFF. Measure voltage on terminal A at ignition control module harness connector with DVOM connected to ground. If DVOM measures above 10V, go to next step. If not, go to step 19).
11. Probe ignition control module harness connector terminal "C" with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to next step. If not, go to step 23).
12. Disconnect ignition control module harness connector. Probe ignition control module harness connector terminal B with DVOM set to AC scale connected to ground. Crank engine. Observe voltage while engine is being cranked. Voltage should be between 1-4 VAC. If voltage is as specified, go to step 20). If voltage is not as specified, go to step 15).
13. Remove distributor cap. Check cap for cracks, moisture, carbon tracks or physical damage. If any of these conditions are present, go to step 34). If cap is okay, go to next step.
14. Crank engine. Observe distributor rotor while engine is being cranked. If rotor did not turn, refer to appropriate REMOVE/INSTALL/OVERHAUL article. If rotor did turn, go to step 33).
15. Turn OFF ignition. Disconnect VCM C3 connector. Check ignition timing signal circuit for an open between the VCM C3 harness connector and ignition control module harness connector terminal B. If circuit is open, go to step 24). If circuit is okay, go to next step.
16. Probe Ignition Control (IC) circuit at VCM C3 connector with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to step 25). If not, go to next step.
17. Ignition ON, engine OFF. Probe Ignition Control (IC) circuit at VCM C3 connector with a test lamp connected to ground. If test lamp is illuminated, go to step 26). If not, go to step 22).
18. Check for an open or shorted to ground ignition positive voltage circuit at terminal A of ignition coil. Repair as necessary. Go to step 37).
19. Check for an open or shorted to ground ignition positive voltage circuit at terminal A of the ignition control module. Repair as necessary. Go to step 37).
20. Check for poor ignition control module connection. If problem was found, go to step 30). If problem was not found, go to step 35).
21. Check for a poor coil connection. If problem was found, go to step 30). If problem was not found, go to step 31).
22. Check for a poor VCM connection. If problem was found, go to step 30). If problem was not found, go to step 36).
23. Repair open ignition control module ground circuit. Go to step 37).
24. Repair open IC circuit between VCM and ignition control module. Go to step 37).
25. Repair grounded IC circuit between VCM and ignition control module. Go to step 37).
26. Repair short to voltage in the IC circuit between the VCM and the ignition control module. Go to step 37).
27. Repair open in tachometer signal circuit between ignition coil and ignition control module. Go to step 37).
28. Repair short to ground in tachometer signal circuit between ignition coil and ignition control module. Go to step 37).
29. Repair short to ground in tachometer input circuit between ignition coil and instrument cluster. Go to step 37).
30. Repair circuit as necessary. Go to step 37).
31. Replace ignition coil. Refer to appropriate REMOVE/INSTALL/OVERHAUL article. Go to step 37).
32. Replace coil wire. Go to step 37).
33. Replace distributor rotor. Go to step 37).
34. Replace distributor cap. Go to step 37).
35. Replace ignition control module. Refer to appropriate REMOVE/INSTALL/OVERHAUL article. Go to step 37).
36. Replace VCM. Go to step 37).
37. Operate vehicle within conditions under which original symptom was noted. If system is not functioning properly, repeat testing from step 1).

Diagnostic Aids

Note. Numbers below refer to the step numbers in diagnostic test.

Note. Battery should be fully charged prior to any tests.

1. Perform the OBD system check before proceeding unless it was already performed.
2. Checks for proper output from the enhanced ignition system. The spark tester requires a minimum of 25,000 volts to operate. This check can be used in case of an ignition miss, because the system may provide enough voltage to run the engine but not enough to operate a spark plug under heavy load.
3. This test separates distributor cap, rotor and ignition wires from ignition coil in order to help identify a secondary ignition system problem.
4. This test checks ignition control module, connections, and wiring.
5. This test begins to determine if the VCM is providing a signal to the ignition control module. If the VCM is not providing a signal to the ignition control module, the problem exists between the ignition control module and the VCM.
6. This test checks for a basic engine mechanical problem.

IDLE SPEED & IGNITION TIMING

Ensure idle speed and ignition timing are set to specifications. For specifications, see SPECIFICATIONS article. For adjustment procedures, see ADJUSTMENTS article.

ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK

Note. Use of Tech 1 scan tool is required to perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK.

The OBD System Check determines

1. If Malfunction Indicator Light (MIL) works.
2. If PCM/VCM is operating and can recognize a fault.
3. If any Diagnostic Trouble Codes (DTCs) are stored.

After performing procedures in PRELIMINARY INSPECTION & ADJUSTMENTS, BASIC FUEL SYSTEM CHECKS and BASIC IGNITION SYSTEM CHECKS, this is the starting point for utilizing the self-diagnostic system for determining computer-related problems. After performing necessary tests as described in the OBD system circuit check, if no codes are indicated and driveability problems still exist, see TESTS W/O CODES article and SCAN TESTER usage in TESTS W/CODES article.

1. Turn ignition on with engine off. Observe Malfunction Indicator Light (MIL). If MIL illuminates, go to next step. If MIL does not illuminate, go to «A-1, MALFUNCTION INDICATOR LIGHT (MIL)»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__a-1-malfunction-indicator-light-mil) .
2. Turn ignition off. Install Tech 1 scan tool and follow scan tool manufacturer's instructions to proceed with test. Turn ignition on. If scan tool displays PCM data, go to next step. If scan tool does not display PCM data, go to A-2, NO SCAN TOOL DATA.
3. If DTC(s) are present, go to step 6). If DTC(s) are not present, go to next step.
4. Attempt to start engine. If engine start and runs, go to next step. If engine does not start or starts and dies, go to «A-3, NO-START DIAGNOSIS»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline) .
5. Compare scan tool engine data with actual control system data values. If parameter is within limits, go to step 7). If parameter is not within limits, go to step 8).
6. Diagnose DTC(s) using appropriate self-diagnostic test. See TESTS W/CODES article.
7. See TESTS W/O CODES article.
8. See SYSTEM/COMPONENT TESTS article.

Scheme 109

A-1, MALFUNCTION INDICATOR LIGHT (MIL)

When vehicle operator initially turns ignition ON, MIL momentarily flashes ON then OFF and then stays ON. When the engine is running and no DTCs are stored, the VCM turns OFF the MIL. Ignition switch supplies battery voltage directly to MIL. VCM controls MIL by providing a ground path through the MIL control circuit to turn OFF MIL.

1. Perform Powertrain OBD System Check. See «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__on-board-diagnostic-obd-system-check) . Turn engine OFF for 15 seconds. Turn ignition ON, engine OFF. If MIL is ON, go to step 2). If MIL is not ON, attempt to start engine. If engine starts, go to step 5). If engine does not start, go to step 6).
2. Install scan tool. Command MIL ON and OFF. If MIL does not function as specified, go to next step. If MIL turns ON and OFF when commanded, allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
3. Turn OFF the ignition. Disconnect black VCM connector. Turn ON ignition. If MIL does not turn ON, go to step 8). If MIL turns ON, go to step 17).
4. Turn OFF ignition. Disconnect black VCM connector. Turn ON ignition. Probe MIL control circuit with a fused jumper connected to ground. If MIL is not ON, go to step 6). If MIL is ON, go to step 11).
5. Inspect fuses 9 (ECM-BATT) and 10 (ECM-I) for an open. If fuses are okay, go to step 7). If fuses are not okay, go to step 8). 6)If fuse in the fused jumper blew, go to step 11). If fuse is okay, go to step 12).
6. Turn OFF ignition. Disconnect black VCM connector. Turn ON ignition. Probe VCM ignition feed circuit (terminal BK18) with a test light to ground. If test light illuminates, go to step 16). If test light does not illuminate, go to step 18).
7. Check for a short to ground in the circuit of the fuse that was open and repair if necessary. Replace faulty fuse. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
8. Check for faulty VCM connections. If problems were found, Repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay. If no problems were found, replace VCM. NOTE: If VCM is faulty, reprogram VCM.
9. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
10. Repair the open in the VCM ignition feed circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
11. 11) Repair short to voltage in MIL control circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
12. Check for an open in MIL control circuit. If control circuit is okay, go to next step. If control circuit is not okay, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
13. Check for an open ignition feed circuit or fuse (gauge 4) to MIL. If no problem was found, go to next step. If circuit was open, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
14. Check for an open MIL bulb. If MIL bulb is okay, go to next step. If bulb is not okay, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
15. Replace the instrument panel. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
16. Turn ignition OFF. Disconnect Gray VCM connector. Probe battery feed circuit (terminal GR21) with a test light connected to ground. If test light illuminates, go to step 19). If test light does not illuminate, go to step 20).
17. Repair short to ground in MIL control circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
18. Repair open in VCM ignition feed circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
19. Check for faulty VCM ground connection at engine block. If ground is okay, go to step 9). If ground is faulty, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
20. Repair open battery feed circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
21. Repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.

An open gauge fuse will cause the entire instrument panel cluster to be inoperative. Check any circuitry that is suspected of causing an intermittent complaint for the following conditions

1. Backed out terminals
2. Improper mating
3. Broken locks
4. Improperly formed or damaged terminals
5. Poor terminal to wiring connections
6. Physical damage to the wiring harness

A-2, NO SCAN TOOL DATA

1. Perform «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__on-board-diagnostic-obd-system-check). After performing OBD system check, go to next step.
2. Turn ignition off for 30 seconds. Turn ignition on with engine off. Observe MIL. If MIL illuminates, go to next step. If MIL does not illuminate, see «A-1, MALFUNCTION INDICATOR LIGHT (MIL)»(/chevrolet/suburban/ix-1991-2001/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-gasoline__a-1-malfunction-indicator-light-mil).
3. Install scan tool. If scan tool communicates with VCM, system is okay. If scan tool does not communicate with VCM, go to next step.
4. Disconnect scan tool. Using DVOM, check voltage between ground DLC terminal No. 2. DVOM should read 5 volts. If voltage is greater than 5 volts, go to step 16). If voltage is 5 volts or less, go to next step.
5. Connect a test light to ground. Using test light, probe DLC terminal No. 16. (Scheme 109) If test light illuminates, go to next step. If test light does not illuminate, go to step 11).
6. Connect test light to battery voltage. Using test light, probe DLC terminals No. 4 and 5. If test light illuminates on both circuits, go to next step. If test light does not illuminate on either circuit, go to step 12).
7. Check cigarette lighter for power. If power is present at cigarette lighter, go to next step. If power is not present at cigarette lighter, go to step 13).
8. Install and check scan tool on another vehicle. If scan tool works, go to next step. If scan tool does not work, go to step 14).
9. Turn ignition off. Using DVOM, check resistance between ground and DLC terminal No. 2. Resistance should be 10 ohms. If resistance is less than 10 ohms, go to step 15). If resistance is 10 ohms or greater, go to step 18).
10. Disconnect Black VCM connector. Using DVOM, check resistance between ground and DLC terminal No. 2. Resistance should be 10 ohms. If resistance is less than 10 ohms, go to step 15). If resistance is 10 ohms or greater, go to step 18).
11. Check fuse No. 7 (AUX PWR). If fuse is faulty, repair short to ground in DLC terminal No. 16 circuit (Orange wire). If fuse is okay, repair open in DLC terminal No. 16 circuit (Orange wire). After repairs, go to step 19).
12. Repair open in affected circuit. After repairs, go to step 19).
13. Repair cigarette lighter circuit. After repairs, go to step 19).
14. Repair or replace faulty scan tool and/or cables. After repairs, go to step 19).
15. Repair short to ground in serial data line (Purple wire). (Scheme 109) After repairs, go to step 19).
16. Disconnect Black VCM connector. Recheck voltage at DLC terminal No. 2. If voltage is greater than 5 volts, go to next step. If voltage is less than 5 volts, go to step 18).
17. Repair short to voltage in serial data line (Purple wire). After repairs, go to step 19).
18. Check for open in serial data line. If circuit is okay, check VCM and DLC connectors. If connectors are okay, replace VCM. Perform VCM relearn procedures. After repairs, go to next step.
19. Using scan tool, select DTC, CLEAR INFO function. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and dies/stalls, go to step 2).
20. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN function. If any DTC(s) are stored, diagnose DTC. See TESTS W/CODES article. If DTC(s) are not present, go to next step.
21. Select CAPTURE INFO, REVIEW INFO function. If additional DTC(s) are displayed, diagnose DTC. See TESTS W/CODES article.

SUMMARY

If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, no trouble codes (or only intermittent ones) were found while performing ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK and driveability problems exist, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnostic procedures.

See also:
• FUEL INJECTION TROUBLE SHOOTING
• FUEL PUMP
• ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK
• A-3, NO-START DIAGNOSIS
• FUEL SYSTEM PRESSURE RELIEF
• A-1, MALFUNCTION INDICATOR LIGHT (MIL)