Engine Controls - Basic Testing Chevrolet Chevy Van G10

INTRODUCTION

The following diagnostic steps can help prevent overlooking simple problems. This is also the place to start diagnosing a no-start condition.

The first step in diagnosing any driveability problem is to verify the problem exists. This may be accomplished by test driving the vehicle under the conditions during which the problem reportedly occurs.

Before entering self-diagnostics, perform a careful and complete inspection of several systems. Most driveability or no-start problems are not related to computerized engine control systems, but are usually simple mechanical, electrical, fuel or vacuum problems. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged or misrouted vacuum hoses.

Before considering the computer system as a possible cause of problems, check ignition high tension wires, fuel supply, electrical connections and vacuum hoses. Failure to check these items can result in improper diagnosis or lost diagnostic time.

Note. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in testing procedures.

VISUAL INSPECTION

Perform a visual inspection of all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Inspect electrical connectors and connections for tight fit and corrosion. Repair as necessary. Inspect all vacuum hoses for proper routing, cuts or pinches. If necessary, see the VACUUM DIAGRAMS article to verify routing and connections. Repair as necessary. Inspect air induction system for possible vacuum leaks.

COMPRESSION

Check engine mechanical condition using a compression gauge, vacuum gauge, or an engine analyzer capable of performing a relative compression test. 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. The exhaust system can be checked with a vacuum gauge or a low pressure (1-5 psi) pressure gauge.

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

1. Check at AIR Pipe - Remove rubber hose at exhaust manifold AIR pipe check valve and remove check valve. Install pressure gauge to hose and nipple via Propane Enrichment Device (J26911). Nipple should be inserted into exhaust manifold AIR pipe.
2. Check at O2 Sensor - Remove O2 sensor. Install backpressure tester in place of O2 sensor. After test is completed, coat O2 sensor threads with anti-seize compound.
3. Diagnosis: 1) Start engine and bring to operating temperature. Increase engine speed to 2000-2500 RPM and note gauge. Reading should not exceed 1.25 psi (.09 kg/cm 2 ). Exhaust system is restricted if specification is exceeded. 2) Check exhaust system for collapsed pipe, heat distress and possible internal muffler failure. If none of these conditions exist, check for restricted catalytic converter. Replace as required.

NO-START DIAGNOSIS

Note. For diesel information, see TESTS W/CODES - DIESEL and TESTS W/O CODES articles.

Definition

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

NO-START - ENGINE CRANKS OKAY (TBI WITH HEI)

Note. Check battery condition, engine cranking speed and fuel supply before performing the following tests.

GENERAL INSPECTION

1. Ensure proper starting procedure is being used.
2. Visually check vacuum hoses for splits, kinks and proper connections, as shown on underhood emission control information label. Check ignition wires for cracking, hardness and proper connections at both distributor cap and spark plugs.
3. Remove spark plugs. Check and replace as necessary.
4. Remove distributor cap and check for moisture, dust, cracks, burns and arcing to ground through coil mounting screws or rotor.
5. Try to turn distributor shaft by hand. Drive gear pin may be broken.
6. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

IGNITION SYSTEM

1. Disconnect tachometer wire at distributor TACH terminal (if equipped). A shorted tachometer or tachometer circuit will not allow vehicle to start.
2. Check for battery voltage at BAT terminal at distributor with ignition on. Repair as necessary.
3. Connect ST-125 spark tester to one end of plug wire and crank engine. If spark is present, check fuel delivery.
4. If spark does not occur, disconnect 4-wire EST connector at distributor. If spark now occurs, replace pick-up coil in distributor.
5. If spark does not occur, reconnect EST connector and check voltage at TACH terminal at distributor with ignition on.
6. If voltage is less than one volt, repair faulty coil connection or replace faulty coil. If voltage is 1-10 volts, replace ignition module and recheck for spark. If spark still does not occur, replace ignition coil.
7. If voltage reading at TACH terminal of distributor is greater than 10 volts, remove and invert distributor cap with wires connected. Fabricate an HEI coil spark tester by trimming a spark plug boot and connecting it to ST-125 spark tester. (Scheme 1) Crank engine.
8. If spark occurs, check cap for cracks, water or other defects. Check pick-up coil connector and ignition coil lead wire colors. Ignition coil with Yellow and Red wires should be used with a Yellow pick-up coil connector. Ignition coil with White and Red wires should be used with a Clear or Black pick-up coil connector.
9. If spark does not occur, turn ignition off and disconnect pick-up coil leads from module. Turn ignition on. With voltmeter connected to distributor TACH terminal and fabricated coil spark tester connected, momentarily touch test light, connected to a remote voltage source (1.5-8.0 volts), to ignition module terminal "P".
10. If voltage at TACH terminal does not drop, check ignition module ground and for open in wires from ignition coil to module. If all is okay, replace ignition module.
11. If voltage at TACH terminal drops, check for spark at spark tester as test light is removed from terminal "P". If spark occurs, check pick-up coil connections and check for 500-1500 ohms resistance at pick-up coil leads. (Scheme 7) Repair as necessary.
12. If spark does not occur, test ignition module with module tester. If module tests okay, check ignition coil wire. If module tester is not available, replace ignition coil and again touch terminal "P". If spark occurs, system is okay. If spark does not occur, reinstall original ignition coil and replace ignition module.

Scheme 1

FUEL SYSTEM

1. Before checking fuel system for a no-start condition, check ignition for adequate spark. Check for proper fuel pump pressure (9-13 psi) and capacity (one pint in 30 seconds). See appropriate BASIC FUEL SYSTEM CHECKS in this article.
2. Crank engine and watch for injector spray. If injector spray occurs, go to step 5). If no spray occurs, disconnect injector harness and check for battery voltage at harness. Battery voltage should be present on one of the injector terminals. If battery voltage is not present, check for blown injector power fuse. If battery voltage is present on both terminals, check for wires shorted to one another.
3. If battery voltage is present on only one terminal, connect injector test light (also called a "noid" light) to injector harness. Crank engine and note light. If light flashes, check for stored ECM codes. See «DIAGNOSTIC CIRCUIT CHECK (GASOLINE)»(/chevrolet/chevy-van-g10/1985-1995/remont/testing-diagnostics/#engine-controls-basic-testing__diagnostic-circuit-check-gasoline) or «DIAGNOSTIC CIRCUIT CHECK (DIESEL)»(/chevrolet/chevy-van-g10/1985-1995/remont/testing-diagnostics/#engine-controls-basic-testing__diagnostic-circuit-check-diesel) in this article. If no codes are present, refer to HARD START symptom in «TESTS W/O CODES»(/chevrolet/chevy-van-g10/1985-1995/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article. If light does not flash, momentarily touch test light from battery voltage to ECM RPM reference terminal (circuit No. 430).
4. Each time test light is removed from ECM RPM reference terminal, injector test light should flash. If test light does not flash, check for open in RPM reference wire, injector drive (ground) circuit, or replace faulty ECM. Prior to replacing ECM, check ECM power and ground circuits.
5. If injector spray occurred while cranking engine, disconnect injector harness and crank engine. If injector spray or leakage occurs, this could cause a no-start condition due to excessive fuel being delivered during cranking. Repair faulty injector or injector seal. If no spray or leakage occurs, refer to HARD START symptom in «TESTS W/O CODES»(/chevrolet/chevy-van-g10/1985-1995/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article.

Scheme 2

Scheme 3

Scheme 4

MODEL 700 TBI UNIT

Place transmission gear selector in park (neutral on manual transmissions). Set parking brake and block drive wheels. Loosen fuel filler cap to relieve tank pressure. Disconnect 3-terminal electrical connector at fuel tank. Start engine and allow to run until engine stalls due to lack of fuel. Crank engine for 3 seconds to remove any fuel pressure left in fuel lines. Fuel lines are now safe for servicing.

MODEL 220 TBI UNIT

Fuel pressure is relieved and drops to zero when ignition is turned off.

FUEL SYSTEM PRESSURE TEST

1. Turn engine off and relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF. Remove air cleaner and plug air cleaner (THERMAC) vacuum port (if equipped). Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658A) and Adapter (J-2968A-85) between throttle body and in-line fuel filter. Start engine and observe fuel pressure reading.
2. Fuel pressure should be 9-13 psi (.6-.9 kg/cm 2 ). Go to step 3) if fuel pressure is present but less than specification. If fuel pressure is greater than specification, go to step 4). If no fuel pressure is evident, check for power to fuel pump relay. If power is available to fuel pump relay, test fuel pump relay. See SYSTEM/COMP TESTS article.
3. If fuel pressure is present during cranking or with fuel pump relay by-passed, but is less than 9-13 psi (.6-.9 kg/cm 2 ), gradually pinch off fuel return line to fuel tank. If fuel pressure remains low, check for a plugged fuel filter or restriction in fuel delivery line. If filter is okay and no restrictions are present, replace fuel pump. If pressure increases to within specification when return line is pinched off, replace fuel pressure regulator. CAUTION: DO NOT pinch off fuel return line completely. DO NOT allow fuel pressure build-up to exceed specification, as damage to fuel pressure regulator may occur.
4. If fuel pressure is present, but is greater than 9-13 psi (.6-.9 kg/cm 2 ), check for restriction in return line to fuel tank. If restriction is not present, replace fuel pressure regulator.
5. After testing, relieve fuel pressure. Remove fuel pressure gauge and connect fuel line to fuel filter. Start vehicle and check for leaks. Remove plug from throttle body vacuum port (if equipped). Install air cleaner.

FUEL PRESSURE RELIEF

Disconnect negative battery cable. Remove fuel filler cap. Since these TBI units contain an internal bleed-down feature, after a short time, system fuel pressure should dissipate.

FUEL PRESSURE CHECK

1. Relieve fuel pressure (if necessary). Remove air cleaner and plug thermal vacuum port on throttle body. When removing fuel line, always use 2 wrenches. Install Fuel Pressure Gauge (J-29658B or BT-8205) and Adapter (29658-85) in fuel line between steel line and flexible hose.
2. Turn ignition off. Apply battery voltage to fuel pump test connector using a 10 amp. fused jumper wire. Observe fuel pressure reading. Fuel pressure should read between 9-13 psi (.6-.9 kg/cm 2 ). If no fuel pressure exist, go to step 3). If fuel pressure is less than 9 psi (.63 kg/cm 2 ), go to step 6). If fuel pressure is greater than 13 psi (.91 kg/cm 2 ), go to step 7).
3. Listen for pump operation at fuel tank. If fuel pump is running, check for plugged in-line fuel filer, plugged pump inlet filter, restricted fuel line or leaking pump rubber coupling. If these are okay, replace in-tank fuel pump.
4. If pump was not operating, disconnect fuel pump relay. Using a 10 amp. fuse jumper wire, connect 12 volts to circuit 120 (Gray wire). If pump now operates, check for faulty connection at relay or faulty fuel pump relay. If pump will not operate, check for faulty in-tank pump or faulty pump ground.
5. If fuel pressure was less than 9 psi (.63 kg/cm 2 ), check for restricted fuel filter or restricted line between in-tank fuel pump and pressure gauge. If fuel filter and lines are okay, go to next step.
6. Disconnect injector connector. Apply battery voltage to fuel pump test connector. Gradually pinch fuel pressure gauge and note pressure. If pressure is greater than 13 psi (.91 kg/cm 2 ), replace fuel pressure regulator. If pressure is less than 9 psi (.63 kg/cm 2 ), check for faulty fuel pump or incorrect part. Check coupling hose and pump inlet filter.
7. Disconnect fused jumper wire from test connector. Disconnect fuel return line quick-connect fitting. Attach 5/16" hose to throttle body side of return line and insert other end of hose in a gasoline container.
8. Using a fused jumper wire, apply battery voltage to fuel pump test connector. If fuel pressure is greater than 13 psi (.91 kg/cm 2 ), check for restricted return line from throttle body. If line is okay, replace fuel pressure regulator. If pressure is between 9-13 psi (.63-.91 kg/cm 2 ), correct or replace fuel return line to fuel tank.

FUEL PUMP RELAY

See RELAYS, SOLENOIDS, MOTORS & MODULES in SYSTEM/COMP TESTS article.

FUEL PUMP RELAY BY-PASS PROCEDURE

See FUEL DELIVERY in SYSTEM/COMP TESTS article.

FIELD SERVICE MODE CHECK

Note. Oxygen sensor may cool off after while engine idles causing system to go into open loop. To restore closed loop mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

This test confirms proper fuel system operation and verifies closed loop operation. Clear codes and perform this test after any repair is completed. When performing this check, always engage parking brake and block DRIVE wheels. Parking brake on FWD models does NOT hold drive wheels.

1) Start engine. With engine running, ground test terminal "B" of the ALDL. In closed loop mode, SERVICE ENGINE SOON light will flash at a rate of one flash per second.

2) In open loop, light will flash at a rate of 2.5 flashes per second. If light is off most of the time, a lean exhaust is indicated. If light is on most of the time, a rich exhaust is indicated.

FUEL PUMP FLOW TEST

1. Disconnect fuel line at fuel filter inlet. Disconnect fuel injection pump electric shut-off solenoid wire (Pink).
2. Crank engine for 15 seconds. Fuel pump should supply approximately 1/2 pint of fuel in 15 seconds. If fuel delivery is not as specified, go to FUEL SYSTEM PRESSURE TEST.

1. Turn engine off. Disconnect fuel line at inlet to fuel filter assembly. Install low pressure fuel gauge to fuel line.
2. Crank or run engine for 10-15 seconds. Fuel pressure should be between 5.8-8.7 psi (.41-.61 kg/cm 2 ). If fuel pressure is not as specified, check fuel lines or fuel tank sending unit for restriction. If fuel lines or sending unit are okay, replace fuel pump.

HEI-EST DISTRIBUTOR

Note. Only basic ignition timing is adjustable in HEI/EST ignition system.

SPARK

1. If factory tachometer is connected to ignition coil tachometer terminal, disconnect it before performing tests. When removing spark plug wire from spark plug, twist and pull on boot, NOT on wire.
2. Using Spark Tester (ST-125), check for spark at coil wire (if applicable) and at each spark plug wire using spark tester. Check spark plug wire resistance on suspect wires. Resistance should be no greater than 30,000 ohms.

IGNITION COIL POWER SOURCE

1. Turn ignition on. Using voltmeter, check voltage between terminal "B" of ignition coil and ground on models with remotely mounted ignition coil.
2. On models equipped with integrally mounted ignition coil, check voltage between BAT terminal and ground at distributor. Battery voltage should exist. If not, check for open circuit, blown ignition fuse or defective ignition switch.

IGNITION COIL RESISTANCE (EXTERNALLY MOUNTED)

1. Remove coil connectors and secondary coil wire. In test "A", use high ohmmeter scale. Resistance value should be very high (infinite). (Scheme 5) If not, replace coil.
2. In test "B", use low ohmmeter scale. Reading should be very low or near zero ohms. If not, replace coil. In test "C", use high ohmmeter scale. (Scheme 5) If there is no continuity, replace coil.

Scheme 5

IGNITION COIL RESISTANCE (INTERNALLY MOUNTED)

1. Turn ignition off. Remove the distributor cap and coil assembly. Turn cap upside down. Set ohmmeter to low scale. Connect leads to coil BAT and TACH terminals. (Scheme 6) If resistance exceeds zero, or nearly zero, replace ignition coil.
2. Set ohmmeter on high scale. Connect one lead to coil secondary terminal and the other lead first to TACH terminal and then to ground terminal. If resistance reading in BOTH instances is infinite, replace ignition coil.

Scheme 6

DISTRIBUTOR PICK-UP COIL SHORT & RESISTANCE CHECKS

1. Disconnect pick-up coil leads from HEI/EST module terminals "N" and "P". Set ohmmeter to middle scale and connect one lead to either pick-up coil lead and the other lead to distributor housing. (Scheme 7) Flex pick-up coil leads by hand to check for intermittent shorts to ground. Reading should be infinity at all times. If not, replace pick-up coil.
2. Connect ohmmeter between both pick-up coil leads. Check for intermittent opens by flexing wires and connectors. Resistance should be 500-1500 ohms. If not, replace pick-up coil.

Scheme 7

TACH PULSE (RPM) SIGNAL

The easiest way to check for a tach pulse signal is using a Scan tester connected to the ALDL of the vehicle. RPM should be indicated on tester when engine is being cranked, or is running. Tach pulse (RPM reference will be indicated as a voltage signal when a DVOM (minimum 10-megohm input impedance) is touched to circuit No. 430 ECM terminal. For circuit and terminal reference, see appropriate schematic in NO-START - ENGINE CRANKS OKAY in this article.

IDLE SPEED & IGNITION TIMING (ALL ENGINES)

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

DIAGNOSTIC CIRCUIT CHECK (GASOLINE)

After performing basic diagnostic procedures listed under 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. The Diagnostic Circuit Check determines if the SERVICE ENGINE SOON light works, if the ECM is operating and can recognize a fault, and if any codes are stored. (Scheme 8)

After performing necessary tests as described in the diagnostic 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 - TBI article.

1. Check SERVICE ENGINE SOON light operation. Turn ignition on with Scan tester not connected, ALDL test terminal not grounded and engine not running. SERVICE ENGINE SOON light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE. If light flashes, go to step 3).
2. Grounding the ALDL test terminal at this time should cause SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. If light goes from bright to dim, this is not considered a code. If this occurs, or light remains on and does not flash Code 12, see A-2, SERVICE ENGINE SOON LIGHT WON'T FLASH CODE 12.
3. If light begins to flash as soon as ignition is turned on, check for a short to ground on the diagnostic test terminal wire between ALDL terminal "B" and ECM terminal No. 5. If circuit is okay, replace ECM.

CHART A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE

1. If SERVICE ENGINE SOON light does not illuminate with ignition on and engine off, attempt to start engine. If engine starts, go to step 3). If engine does not start, check fusible links at battery. Also, check ECM fuse. If fusible links or ECM fuse are blown, repair short to ground.
2. If fusible links and ECM fuse are okay, turn ignition on and check power circuits to ECM, including keep alive memory and ignition feed. See appropriate diagram in WIRING DIAGRAMS article for power terminal identification. If power is not available to power terminals of ECM, check for opens in power circuits. If power is available to ECM power terminals, check for poor ECM ground circuits, or replace faulty ECM.
3. If engine starts and SERVICE ENGINE SOON light does not illuminate, turn ignition off. Disconnect ECM connectors. Turn ignition on and jumper ECM SERVICE ENGINE SOON light driver terminal to ground using a test light. See appropriate diagram in WIRING DIAGRAMS article for power terminal identification.
4. If light is now on, repair light driver terminal connections at ECM or replace faulty ECM. If light stays off when test light is used to ground light driver terminal, check for blown instrument panel fuse, faulty bulb, open in light driver circuit between ECM and bulb, driver circuit shorted to voltage, or an open in the ignition feed to the SERVICE ENGINE SOON light.

CHART A-2, SERVICE ENGINE SOON LIGHT WON'T FLASH CODE 12

1. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If SERVICE ENGINE SOON light is on, check for short to ground in light driver circuit between light and ECM driver terminal. See appropriate diagram in WIRING DIAGRAMS article for terminal identification.
2. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off. Using a DVOM, check voltage at ALDL test terminal "B". If voltage is greater than 9 volts, check for a short to voltage on ALDL terminal "B" wire between ECM and ALDL connector.
3. If voltage is less than 5 volts, backprobe appropriate ECM terminal with DVOM. See appropriate diagram in WIRING DIAGRAMS article for terminal identification. If 5-6 volts is now present, repair open or short in wire between ECM and ALDL terminal "B". If voltage is 5-6 volts, go to step 4).
4. If voltage at terminal "B" of ALDL connector is 5-6 volts, jumper that wire terminal at ECM to ground. If SERVICE ENGINE SOON light now flashes a Code 12, and ALDL terminal "A" was originally used when grounding terminal "B" at the beginning of the diagnostic circuit check, check terminal "A" for open circuit.
5. If SERVICE ENGINE SOON light does not flash when ECM end of terminal "B" wire is jumpered to ground, check PROM/MEM-CAL for proper installation. If installed correctly, replace ECM, using original PROM/MEM-CAL. Repeat diagnostic circuit check. If Code 12 still does not flash, replace ECM. Replace PROM/MEM-CAL only after replacing ECM, as PROM/MEM-CAL is not likely to be at fault.

Outlining Diagnostic Circuit Check Flow Chart. Scheme 8

Scheme 9

Circuit Description

This chart assumes that battery conditions and engine cranking speed are OK, and there is adequate fuel in tank. This chart should be used on engines with Model 700 throttle body.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Description

Test Number 1) A "Service Engine Soon" light "ON" is a basic test to determine if there is a 12 volt supply and ignition 12 volts to ECM. No ALDL may be due to an ECM problem and CHART A-2 will diagnose ECM. If TPS is over 2.5 volts the engine may be in the clear flood mode which will cause starting problems. If coolant sensor is below -30°C, the ECM will provide fuel for this extremely cold temperature which will severely flood the engine.

Test Number 2) Voltage at spark plug is checked using spark tester tool ST-125 (J 26792) or equivalent. No spark indicates a basic ignition problem.

Test Number 3) While cranking engine there should be no fuel spray with injector disconnected. Replace an injector if it sprays fuel or drips like a leaking water faucet.

Test Number 4) Use an injector test light like J 34730, BT-8329A or equivalent, to test injector circuit. A blinking light indicates the ECM is controlling the injector.

Test Number 5) This test will determine if there is fuel pressure at the injector and that the injector is operating.

Diagnostic Aids

If no trouble is found in fuel pump circuit or ignition system and the cause of a "Engine Cranks But Will Not Run" has not been found, check for the following

1. Fouled spark plugs.
2. EGR valve stuck open.
3. Low fuel pressure. See CHART A-6.
4. Water or foreign material in the fuel system.
5. A ground circuit 423 (EST) may cause a "No Start" or a "Start then Stall" condition.
6. Basic engine problem.

Scheme 10

Scheme 11

This chart assumes that battery conditions and engine cranking speed are OK, and there is adequate fuel in tank. This chart should be used on engines with Model 220 throttle body.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Number 1) A "Service Engine Soon" light "ON" is a basic test to determine if there is a 12 volt supply and ignition 12 volts to ECM. No ALDL may be due to an ECM problem and CHART A-2 will diagnose ECM. If TPS is over 2.5 volts the engine may be in the clear flood mode which will cause starting problems. If coolant sensor is below -30°C, the ECM will provide fuel for this extremely cold temperature which will severely flood the engine.

Test Number 2) Voltage at spark plug is checked using spark tester tool ST-125 (J 26792) or equivalent. No spark indicates a basic ignition problem.

Test Number 3) While cranking engine there should be no fuel spray with injector disconnected. Replace an injector if it sprays fuel or drips like a leaking water faucet.

Test Number 4) Use an injector test light like BT-8320 or equivalent, to test injector circuit. A blinking light indicates the ECM is controlling the injector.

Test Number 5) This test will determine if there is fuel pressure at the injector and that the injector is operating.

If no trouble is found in fuel pump circuit or ignition system and the cause of a "Engine Cranks But Will Not Run" has not been found, check for the following

1. Fouled spark plugs.
2. EGR valve stuck open.
3. Low fuel pressure. See CHART A-6.
4. Water or foreign material in the fuel system.
5. A ground circuit 423 (EST) may cause a "No Start" or a "Start then Stall" condition.
6. Basic engine problem.

Scheme 12

Scheme 13

Scheme 14

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Number 1) This test will determine if ignition module is generating a reference pulse, if wiring is at fault or if ECM is at fault. By touching and removing a test light, connected to 12 volts, to circuit 430, a reference pulse should be generated. If injector test light blinks, ECM and wiring are OK.

Test Number 2) This step tests for 12 volts to injector. It will also determine if there is a short to voltage on ECM side of circuit.

Test Number 3) This test checks for continuity to ECM.

Scheme 15

Scheme 16

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem. If both injector circuits fail to blink when tested, diagnose one injector circuit at a time.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Number 1) This test will determine if ignition module is generating a reference pulse, if wiring is at fault or if ECM is at fault. By touching and removing a test light, connected to 12 volts, to circuit 430, a reference pulse should be generated. If injector test light blinks, ECM and wiring are OK.

Test Number 2) This step tests for 12 volts to injector. It will also determine if there is a short to voltage on ECM side of circuit.

Test Number 3) This test checks for continuity to ECM.

Scheme 17

Scheme 18

When ignition switch is turned "ON," the ECM will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving distributor reference pulses. If there are no reference pulses, the ECM will shut "OFF" the fuel pump within two seconds after ignition "ON" or engine stops.

The pump will deliver fuel to the TBI unit where the system pressure is controlled to about 9-13 psi. Excess fuel is then returned to the fuel tank.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Number 1) Turns "ON" the fuel pump if circuit 120 wiring is OK. If the pump runs, it may be a fuel pump relay circuit problem, which the following steps will locate.

Test Number 2) The next two steps check for power and ground circuits to the relay.

Test Number 3) Determines if ECM can control the relay.

Test Number 4) The oil pressure switch serves as a backup for the fuel pump relay to help prevent a "no start" situation. If the fuel pump relay was found to be inoperative, the oil pressure switch circuit should also be tested to determine why it did not operate the fuel pump.

Scheme 19

Scheme 20

When ignition switch is turned "ON," the ECM will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving distributor reference pulses. If there are no reference pulses, the ECM will shut "OFF" the fuel pump within two seconds after ignition "ON" or engine stops.

The pump will deliver fuel to the TBI unit where the system pressure is controlled to about 9-13 psi. Excess fuel is then returned to the fuel tank.

A fuel module is used on all 7.4L, G van with 5.7L, and all other 5.7L over 8500 GVW engines to correct a hot restart vapor (vapor lock) during a high ambient condition. It is designed to over-ride the ECM two second pump operation and will run the fuel pump for twenty seconds at initial ignition "ON".

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Number 1) This procedure applies direct voltage to run the fuel pump. If the pump runs, it may be a fuel pump relay circuit problem which the following steps will locate.

Test Number 2) This step checks voltage from the battery and the ground circuit to the relay.

Test Number 3) This test determines if there is voltage from the ECM, terminal "A1", to terminal "D" on the relay connector.

Test Number 4) This completes the fuel pump relay circuit but if this diagnosis was used because the engine would not run then oil pressure switch should also be diagnosed.

An inoperative fuel module may be the cause of a hot stall/no start condition. Check for power and ground circuit to the fuel module and a complete circuit to the pump from terminal "A". If OK, and the pump does not run for the specified 20 seconds at initial ignition "ON," replace fuel module.

Note. Figs. (Scheme 21) -24 below are Courtesy of General Motors Corp.

Chart A-5 Schematic, Fuel Pump Ckt Diagnosis (C, K, R, V, G, P & GP Series) One Fuel Tank. Scheme 21

Flow Chart A-5, Fuel Pump Ckt Diagnosis (C, K, R, V, G, P & GP Series) One Fuel Tank. Scheme 22

Chart A-5 Schematic, Fuel Pump Ckt Diagnosis (R & V Series) Two Fuel Tanks. Scheme 23

Flow Chart A-5, Fuel Pump Ckt Diagnosis (R & V Series) Two Fuel Tanks. Scheme 24

When the fuel pump is running, fuel is delivered to the injector(s) and then to the regulator where the system pressure is controlled to about 9-13 psi. Excess fuel is then returned to the fuel tank.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Number 1) Pressure, but less than 9 psi falls into two areas

1. Regulated pressure but less than 9 psi - Amount of fuel to injector OK but pressure is too low. System will be lean running and may set Code 44. Also, hard starting cold and poor overall performance.
2. Restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 9 psi at idle will not be driveable. However, if the pressure drop occurs only while driving, the engine could surge and stop when pressure is too low.

Test Number 2) Restricting the outlet side of the gage allows the pump to develop its maximum pressure (dead head pressure). With battery voltage applied to the pump test terminal, fuel pressure should rise to 13-15 psi as the pressure gage outlet hose is gradually pinched.

Test Number 3) This test determines if high fuel pressure is due to a restricted fuel return line or a throttle body pressure regulator problem.

1. If the vehicle is equipped with a fuel module, the module must be disconnected before performing the fuel system pressure test.
2. Fuel system is under pressure. To avoid fuel spillage, see FUEL SYSTEM PRESSURE RELIEF (GASOLINE) in «REMOVE/INSTALL/OVERHAUL»(/chevrolet/chevy-van-g10/1985-1995/remont/removal-installation/#engine-controls-removeinstalloverhaul) article.
3. On V6 or V8 engines, fuel pressure drops to almost zero psi after pump shuts "OFF".

Scheme 25

Scheme 26

DIAGNOSTIC CIRCUIT CHECK (DIESEL)

Note. The 6.2L with light duty emissions is equipped with a self- diagnostic system. For information on the diagnostic circuit check and retrieving codes, see TESTS W/CODES - DIESEL article in this section.

PRELIMINARY INSPECTION

1. Check all vacuum hoses for correct routing, restrictions, cuts, or other damage. Inspect hoses see beneath the air cleaner assembly and other engine components.
2. Inspect all wires in engine compartment for correct and good connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed CAREFULLY and THOROUGHLY as it can often lead to fixing a problem without further diagnosis.

DIAGNOSTIC PROCEDURE

1. Ensure all engine systems not related to Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all other non-DEC system related problems have been repaired. This determines whether DEC system and ECM are working properly.
2. If trouble codes are displayed (other than Code 12), determine if codes are "hard" or "intermittent" trouble codes. A hard code is one which is present when working on vehicle and condition still exists. Hard codes will cause the SERVICE ENGINE SOON light to come on.
3. An intermittent code is one which does not reset itself and is not present while working on vehicle. An intermittent code is often caused by loose connections. The SERVICE ENGINE SOON light will go out 10 seconds after fault goes away.

Scheme 27

SUMMARY

If no problems were found while performing test procedures in this article, no trouble codes (or only intermittent ones) were found while performing either DIAGNOSTIC CIRCUIT 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 diagnosis procedures.

See also:
• TESTS W/CODES - DIESEL
• TESTS W/O CODES
• TESTS W/CODES - TBI
• REMOVE/INSTALL/OVERHAUL
• DIAGNOSTIC CIRCUIT CHECK (GASOLINE)
• DIAGNOSTIC CIRCUIT CHECK (DIESEL)