Engine Controls - Tests W/o Codes GMC Sonoma Syclone

INTRODUCTION

Before diagnosing symptoms or intermittent faults, perform steps in BASIC TESTING article and appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE Section. Use this article to diagnose driveability problems if a hard fault code is not present or if vehicle is not equipped with a self-diagnostic system.

Symptom checks are intended to direct the technician to malfunctioning component(s) for further diagnosis. A symptom should lead to either specific component or system testing or an adjustment specification. Use intermittent test procedures to locate driveability problems which DO NOT occur when the vehicle is being tested.

Note. For specific testing procedures, see I - SYS/COMP TESTS article. For specifications, see ADJUSTMENTS or SPECIFICATIONS article in the ENGINE PERFORMANCE Section.

SYMPTOMS (GASOLINE)

Note. See SYMPTOMS (DIESEL) if necessary.

HOW TO USE THIS SECTION

Use this portion of article only AFTER performing these checks

1. Ensure on-vehicle diagnostics work (if equipped). Perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article.
2. Ensure ECM and SERVICE ENGINE SOON light function properly.
3. Ensure no trouble codes, except intermittent ones, are stored.
4. Ensure fuel control system operates properly. Perform FIELD SERVICE MODE CHECK inF - BASIC TESTING article.
5. Perform fuel system pressure test.
6. Visually inspect all systems.

After performing these checks, verify customer complaint, and locate correct symptom from among those listed in this article. Not all items listed under each symptom apply to all models and systems. These procedures normally recommend testing of a system or component on vehicle, such as EGR, EST, TCC, etc. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section for specific testing procedures.

Note. If ECM displays data but engine fails to start, see NO-START DIAGNOSIS in BASIC TESTING article in the ENGINE PERFORMANCE Section.

SYMPTOM DEFINITION

Engine cranks okay, but does not start for a long time. Engine eventually starts, and may run okay or die immediately.

POSSIBLE CAUSES & CORRECTIONS

1. Check fuel pump relay. Connect test light between fuel pump test terminal and ground or between fuel pump connector and ground. Turn ignition on. Light should illuminate for 2 seconds. If not, see FUEL PUMP RELAY in I - SYS/COMP TESTS article. For location of fuel pump test connector, see COMPONENT LOCATIONS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.
2. Check for poor fuel quality or water-contaminated fuel.
3. Ensure TPS does not stick or bind.
4. Check EGR valve operation.
5. Check for a leaking injector. Disconnect injector electrical connector at injector. Crank engine, and watch for fuel leakage.
6. Ensure coolant sensor circuit resistance or coolant sensor resistance is not too high. See SENSOR RANGE CHARTS article or CODE 15 chart in G - TESTS W/ CODES article in the ENGINE PERFORMANCE Section.
7. Check ignition system for a worn distributor shaft, bare or shorted wires, incorrect pick-up coil resistance, loose ignition coil ground or moisture in distributor cap. Using Spark Tester (ST-125), check for adequate spark.
8. Check for shorts by misting plug wires with water.
9. Remove spark plugs. Check for wet plugs, cracks, improper gap, burned electrodes or heavy carbon deposits.
10. Check for correct fuel pressure at all speeds.
11. Check for a faulty in-tank fuel pump check valve. A faulty check valve allows fuel in lines to drain back to tank after engine is stopped.
12. Ensure correct PROM is installed in vehicle. Check with dealer for latest application information.
13. Check for restricted exhaust system.

Engine starts okay but dies after brief idle, as soon as any load is placed on engine (such as turning on air conditioner or engaging transmission) or on initial drive-away.

1. Ensure hot air tube is connected to air cleaner.
2. Check thermostatic air cleaner for proper operation.
3. Check Idle Air Control (IAC) system for proper operation.
4. Check PCV valve for proper operation.
5. Unplug MAP sensor. ECM substitutes a default value for sensor signal. If stall condition is eliminated, replace sensor.
6. Check EGR system for proper operation.
7. If stall occurs when air conditioner is turned on, check for air conditioner clutch signal to ECM terminal. Voltage at A/C terminal of ECM should be battery voltage when air conditioner compressor clutch is engaged. A high voltage surge due to a shorted compressor clutch diode could cause ECM shutdown.
8. Check for an overcharged A/C system.
9. Check for plugged or restricted fuel lines.
10. If engine starts but then immediately stalls, open distributor by-pass circuit. If engine now starts and runs okay, replace distributor pick-up coil.
11. Using Spark Tester (ST-125), check for a weak spark from ignition coil.

Momentary lack of response when accelerator is pushed down. Can occur at any vehicle speed. Usually occurs after a stop.

1. Visually check vacuum hoses for splits, kinks and improper connections, as shown on Vehicle Emission Control Information label.
2. Check ignition wires for cracking, hardness and improper connections at both distributor cap and spark plugs.
3. Check wires for pinches, cuts and improper connections.
4. Ensure fuel pressure is correct in all speed ranges. Also check for poor fuel quality or water-contaminated fuel.
5. Check for fouled spark plugs.
6. Ensure correct PROM is installed in vehicle. Check with dealer for latest application information.
7. Ensure TPS does not bind or stick.
8. Ensure initial ignition timing is properly set.
9. Ensure ECM-controlled idle speed is correct.
10. Check EGR system for proper operation.
11. Disconnect fuel injector electrical connectors. Crank engine, and check for injector leaks.
12. Check for an open in HEI ground circuit.
13. Check canister purge system for proper operation.
14. Check charging system output. Repair charging system if voltage is less than 9 volts or more than 16 volts.

Engine power varies under steady throttle or cruise. Feels like vehicle speeds up and slows down even though accelerator pedal position remains constant.

1. Check thermostatic air cleaner damper door for proper operation.
2. Ensure park/neutral switch is properly adjusted.
3. Check for intermittent open or short to ground in Torque Converter Clutch (TCC) or HEI by-pass circuits.
4. Check canister purge system for proper operation.
5. Check ESC system for proper operation.
6. Check EGR system for proper operation.
7. Ensure initial ignition timing is properly set.
8. Using Spark Tester (ST-125), check for adequate spark output
9. Check O2 sensor for lead or RTV sealant contamination.Such contamination causes a false high voltage signal to ECM, which responds by leaning air/fuel ratio.
10. Check in-line fuel filter. Replace if dirty or clogged.
11. Check fuel for water contamination. Ensure fuel system pressure is correct at all engine speeds.
12. Remove spark plugs, and check for wet plugs, cracks, improper gap, burned electrodes or heavy carbon deposits. Also check condition of distributor cap, rotor and spark plug wires.
13. Check A/C for excessive charge.
14. Check for restricted exhaust system.

Engine delivers less power than expected. Little or no increase in speed when accelerator is pushed down.

1. Check air filter and fuel filter. Replace if necessary. Check for incorrect fuel pressure.
2. Check injector wires for short to ground at air cleaner.
3. Check thermostatic air cleaner damper door for proper operation.
4. Ensure initial ignition timing is properly set.
5. Check TCC system for proper operation.
6. Check ECM grounds.
7. Check ESC system for excessive retard.
8. Check EST system for proper operation.
9. Ensure EGR valve is not always open.
10. Check exhaust system for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter. See EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION in BASIC TESTING article.
11. Check charging system output. Repair charging system if voltage is less than 9 volts or more than 16 volts.
12. Check MAP sensor output.
13. Using Spark Tester (ST-125), check for available secondary voltage.
14. Check engine valve timing and compression.
15. Check engine for a worn camshaft.

Fuel ignites in intake manifold or in exhaust system, making a loud popping noise.

1. Ensure EGR valve is not always open.
2. Check for proper valve timing.
3. Check for engine vacuum leaks. Ensure engine is tuned to specifications.
4. Check for faulty air injection diverter valve or check valve.
5. Check engine for sticking or leaking valves.
6. Check for fuel or water in vacuum hose to MAP sensor. Also check for restricted hose.
7. Using Spark Tester (ST-125), check available output voltage of ignition coil.
8. Check for crossfire between spark plugs, distributor cap and spark plug wires.
9. Check for an intermittent ignition system problem.
10. Check for erratic spark timing or distributor reference (RPM) signal.
11. Ensure initial ignition timing is properly set.

Cuts out, misses is defined as a steady pulsation or jerking following engine speed, usually more pronounced as engine load increases. Exhaust may have a steady spitting sound at idle or low speed. Perform careful visual inspection as described in BASIC TESTING article.

1. Check ignition wires for short or faulty insulation.
2. Check distributor cap for moisture, dust or cracks. Finely mist spark plug wires with water to check for shorts.
3. Connect Spark Tester (ST-125) to spark plug, and check for adequate spark.
4. Check ignition system for faulty grounds.
5. Ensure EST wiring harness is not routed too close to wiring. EST wiring harness routed too close to wiring may cause induced voltage signals.
6. Check ignition coil connections.
7. Remove spark plugs, and check for incorrect heat range, wear, cracks, wetness, improper gap or heavy deposits.
8. Check for poor or contaminated fuel.
9. Check for improper fuel pressure.
10. Ensure EGR valve does not stick open.
11. Check ECM for proper ground circuits.
12. Ensure TPS does not stick or bind. TPS voltage should be less than 1.25 volts at idle.
13. Check for proper pick-up coil (HEI distributor) resistance.
14. Check for restricted exhaust system. See EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION in BASIC TESTING article.
15. Check for bent push rods, broken valve springs or worn camshaft lobes.

MISFIRE ISOLATION

1. Start engine. Disconnect IAC motor. Using insulated pliers, remove one spark plug wire from a spark plug, and ground it against engine.
2. Note engine RPM as wire is grounded. Reconnect spark plug wire. Repeat procedure for all cylinders. Stop engine, and reconnect IAC motor.
3. If engine speed dropped equally (within 50 RPM) on all cylinders, refer to ROUGH, UNSTABLE OR INCORRECT IDLE symptom. If there was no engine RPM drop or no excessive variation on one or more cylinders, check spark on respective cylinder(s).

Engine runs unevenly at idle. If bad enough, vehicle will shake. Idle RPM may vary. Engine idles at incorrect RPM.

1. Ensure throttle linkage and/or TPS do not stick or bind.
2. Ensure initial ignition timing is properly set.
3. Check engine idle speed (both base and ECM idle).
4. Check Idle Air Control (IAC) system. Check for foreign material in IAC bore. See appropriate Idle Air Control (IAC) Chart in «SYSTEM/COMPONENT TESTS»(/gmc/sonoma-syclone/1991-1992/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article.
5. Check EGR system for proper operation.
6. Check park/neutral switch circuit. Ensure park/neutral switch is properly adjusted.
7. Check power steering pressure switch circuit.
8. Check exhaust system for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter. See EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION in BASIC TESTING article.
9. If rough idle occurs only when engine is hot, check PCV valve for proper operation. Check evaporative emission control system. Check for proper spark plug gap and engine compression.

Engine starts but will not run at idle. Engine runs if accelerator is held at part throttle.

1. Problem is most likely in Idle Air Control (IAC) system. See DIAGNOSTIC AIDS in CODE 35 chart in appropriate G - TESTS W/ CODES article.
2. Check EGR system.
3. Check park/neutral switch.
4. Disconnect MAP sensor. If condition is corrected, replace sensor.

Fuel economy, as measured by an actual road test, is noticeably lower than expected. Current fuel economy is noticeably lower than previous fuel economy.

1. Check thermostatic air cleaner damper door for proper operation. Also check for a clogged air filter.
2. Check cooling system thermostat for proper heat range and operation.
3. Check coolant sensor for shift in calibration. See COOLANT TEMPERATURE RESISTANCE VALUES table in SENSOR RANGE CHARTS article.
4. Ensure speedometer is properly calibrated.
5. Check engine compression.
6. Check for dragging brakes.
7. Ensure A/C is not always on.
8. Ensure initial ignition timing is properly set. Check EST and ESC for proper operation.
9. Check TCC for proper operation.
10. Ensure air pump output shifts to catalytic converter upon signal from TVS. Check for faulty electrical and/or vacuum circuits.
11. Check exhaust system for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter. See EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION in BASIC TESTING article.
12. Check O2 sensor for silicone or lead contamination.
13. Remove spark plugs, and check for wet plugs, cracks,improper gap, burned electrodes or heavy carbon deposits.

Engine continues to run (but very rough) after ignition is turned off. If engine runs smoothly, check ignition switch.

1. Check for binding throttle linkage.
2. Check for leaking injectors.
3. Check IAC system. See DIAGNOSTIC AIDS in CODE 35 chart in appropriate G - TESTS W/ CODES article.
4. Check engine for overheating.

A mild to severe ping, usually worse during acceleration. Engine makes sharp metallic knocks which change with degree of acceleration.

1. Check for obvious overheating problems.
2. Ensure initial timing is correct.
3. Check TPS adjustment and operation.
4. Check fuel system for low pressure or volume. Also check for induction air leaks.
5. Ensure ESC system is operating properly.
6. Ensure EGR valve is not always open.
7. Ensure TCC system is operating properly.
8. Remove carbon from engine with top engine cleaner.
9. If excessive carbon exists in combustion chamber, check for excessive oil burning due to leaking valve guide seals.
10. Check for incorrect basic engine parts such as camshaft, cylinder heads and pistons.
11. Ensure correct PROM is installed in vehicle. Check with dealer for latest application information.

Vehicle fails emission test. Vehicle may also emit "rotten egg" smell (hydrogen sulfide) from exhaust pipe.

1. Check for lead contamination of catalytic converter. Check for removal/tampering at restrictor in fuel filler neck.
2. Ensure air is not being diverted to exhaust manifold and is being diverted to catalytic converter during normal engine operation.
3. If emission test shows excessive carbon monoxide (CO) and hydrocarbons (HC) emissions and vehicle emits odor, check all systems and components which could cause engine to run rich. See DIAGNOSTIC AIDS in CODE 45 chart in appropriateG - TESTS W/ CODES article.
4. If emission test shows excessive oxides of nitrogen (NOx) emissions, check all systems and components which could cause engine to run lean or too hot. See DIAGNOSTIC AIDS in CODE 44 chart in appropriate G - TESTS W/ CODES article.

ENGINE WILL NOT START

ENGINE WILL NOT START

ENGINE STALLS AT IDLE

ENGINE STALLS AT IDLE

ENGINE STARTS, IDLES ROUGH WITHOUT UNUSUAL NOISE OR SMOKE

ENGINE STARTS, IDLES ROUGH WITHOUT UNUSUAL NOISE OR SMOKE

ENGINE STARTS AND IDLES WITH EXCESSIVE NOISE AND/OR SMOKE

ENGINE STARTS AND IDLES WITH EXCESSIVE NOISE AND/OR SMOKE

ENGINE IDLES BUT MISFIRES ABOVE IDLE

ENGINE IDLES BUT MISFIRES ABOVE IDLE

ENGINE WILL NOT IDLE

ENGINE WILL NOT IDLE

FUEL LEAKS WITH NO OTHER ENGINE MALFUNCTION

FUEL LEAKS WITH NO OTHER ENGINE MALFUNCTION

LOW ENGINE POWER

LOW ENGINE POWER

"RAPPING" NOISE FROM CYLINDERS

"RAPPING" NOISE FROM CYLINDERS

EXCESSIVE NOISE WITH BLACK SMOKE

EXCESSIVE NOISE WITH BLACK SMOKE

ENGINE WILL NOT SHUT OFF WITH KEY

ENGINE WILL NOT SHUT OFF WITH KEY

SYMPTOMS/CAUSES

To trouble shoot following symptoms, refer to appropriate number under PROBABLE CAUSES.

SYMPTOM TO PROBABLE CAUSES (1)

PROBABLE CAUSES

INTERMITTENT PROBLEM DIAGNOSIS

Intermittent fault testing requires duplication of circuit or component failure in order to identify fault. These procedures may lead to computer recording a fault code which may help diagnosis.

If problem vehicle does not produce fault codes, use a DVOM to monitor voltage or resistance values while attempting to reproduce conditions which will create an intermittent fault. A change in status on DVOM indicates a fault has been located.

When using a voltmeter to pinpoint faults, monitor voltage reading with ignition on or vehicle running. A change in status on voltmeter while performing test procedure indicates area of fault. See TEST PROCEDURES under INTERMITTENTS.

When using an ohmmeter to detect problems in circuit, monitor circuit resistance (ohms) with ignition switch in OFF position or with battery disconnected. A change in ohmmeter reading while performing test procedure indicates area of fault. See TEST PROCEDURES under INTERMITTENTS.

INTERMITTENT SIMULATION

Following methods may reproduce conditions which create an intermittent fault so fault may be identified during testing

1. Applying light vibration to components.
2. Heating a component.
3. Wiggling or bending a wiring harness.
4. Applying humidity to a component.
5. Removing or applying a vacuum supply source.

Monitor circuit/component voltage or resistance while attempting to simulate intermittent. If vehicle is running, monitor for self-diagnostic codes. Use results of these tests to identify a faulty component or an area which should be closely checked for problem.

SERVICE ENGINE SOON light comes on but does not stay on. A stored code may or may not exist.

To determine possible causes of an intermittent SERVICE ENGINE SOON light operation

1. Check for poor mating of one connector to another. Terminals may not be fully seated. Check for improperly formed or damaged terminals. Check wire-to-terminal connections.
2. Check for poor connection from ignition coil to ground or arcing at spark plug wires or plugs.
3. Check wire from SERVICE ENGINE SOON light to ECM for short to ground.
4. Check wire from ALDL "test" terminal for intermittent short to ground.
5. Check for poor connections in ECM ground terminals.
6. Check for loss of trouble code memory. To check code, disconnect TPS, and run engine at idle until SERVICE ENGINE SOON light comes on. Code 22 should be stored and retained in memory when ignition is turned off. If not, ECM is faulty.
7. Check for electrical system interference caused by a defective relay or an ECM-driven solenoid or switch which may cause a sharp electrical surge. This type of problem will normally occur when faulty component is operated.
8. Check for aftermarket parts which may not have been produced to manufacturers' specifications. Solenoids without original -equipment diodes for circuit protection and HEI-EST module or voltage regulator using transistors instead of silicone -chip circuitry may possibly cause voltage surges (up to 300 volts) in ECM wiring, causing temporary ECM shutdown. ECM shutdown is a normal response to system overvoltage (over 16 volts on most models). ECM repowers when condition ceases to exist. A rapid shutdown and repower could cause a flickering SERVICE ENGINE SOON light with no codes set in memory.
9. Check for improper installation of electrical accessories such as auxiliary lights or 2-way radios.
10. Ensure EST wires are kept away from spark plug wires, distributor wires, distributor housing, ignition coil and generator. Ensure ground wire from ECM to distributor or ignition module is connected to a good ground.
11. Check for intermittent short to ground on terminal "B"(diagnostic enable) of ALDL or in SERVICE ENGINE SOON light circuit.
12. On vehicles not equipped with a driver information center, use Scan tester to check for intermittent wiring problem. See SCAN TESTER USAGE in appropriate G - TESTS W/ CODES article.

See also:
• SYSTEM/COMPONENT TESTS
• SYMPTOMS (DIESEL)