Engine Controls - Tests W/o Codes: Diagnosis Chevrolet Sportvan G20

SYMPTOMS (GASOLINE)

Note. Also see SYMPTOMS (DIESEL) in this article.

SYMPTOM DEFINITION

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

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

Possible Cause & Correction Check the following

1. Ensure hot air tube is connected to air cleaner.
2. Check for proper operation of thermostatic air cleaner.
3. Check for proper operation of Idle Air Control (IAC) system.
4. Check PCV valve for proper operation.
5. Unplug MAP sensor. ECM will substitute 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 then 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 when taking off from a stop.

Possible Cause & Correction Check the following

1. Visually check vacuum hoses for splits, kinks and proper connections, as shown on Vehicle Emission Control Information label.
2. Check ignition wires for cracking, hardness and proper connections at both distributor cap and spark plugs.
3. Check wires for pinches, cuts, and proper 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. Check for a binding or sticking TPS.
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 without changing position of accelerator pedal.

Possible Cause & Correction Check the following

1. Check operation of thermostatic air cleaner damper door.
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 for proper operation of canister purge system.
5. Check for proper operation of ESC system.
6. Check for proper operation of EGR system.
7. Ensure initial ignition timing is properly set.
8. Check for adequate spark output using Spark Tester (ST-125).
9. Check O2 sensor for lead or RTV sealant contamination. This will cause a false high voltage signal to ECM. ECM will respond by leaning air/fuel ratio.
10. Check in-line fuel filter and 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.

Possible Cause & Correction Check the following

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 for proper operation of thermostatic air cleaner damper door.
4. Ensure initial ignition timing is properly set.
5. Check for proper operation of TCC system.
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 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.

Possible Cause & Correction - Check the following

1. Ensure EGR valve is not open all the time.
2. Check for proper valve timing.
3. Check for engine vacuum leaks and/or engine not 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.
11. Ensure initial ignition timing is properly set.

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

Possible Cause & Correction Check the following

1. Check ignition wires for short or faulty insulation.
2. Check distributor cap for moisture, dust or cracks. Spray spark plug wires with a fine mist of 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 which may cause induced voltage signals.
6. Check ignition coil connections.
7. Remove spark plugs and check for correct heat range, wear, cracks, wetness, improper gap or heavy deposits.
8. Check for poor or contaminated fuel.
9. Check for improper fuel pressure.
10. Check for EGR valve sticking open.
11. Check ECM for proper ground circuits.
12. Check TPS for sticking or binding. 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 Page 3 in BASIC TESTING article.
15. Check for bent push rods, broken valve springs or worn camshaft lobes.

Note. Grounding spark plug wire for extended periods may cause catalytic converter overheating.

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

Possible Cause & Correction Check the following

1. Ensure throttle linkage and/or TPS are not sticking or binding.
2. Ensure initial ignition timing is properly set.
3. Check engine idle speed, both base idle and ECM idle.
4. Check Idle Air Control (IAC) system. Check for foreign material in IAC bore. See DIAGNOSTIC AIDS in CODE 35 chart in appropriate TESTS W/CODES article.
5. Check for proper operation of EGR system.
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 BASIC TESTING article.
9. If rough idle only occurs 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 will run if accelerator is held at part throttle.

Possible Cause & Correction Check the following

1. Problem is most likely in Idle Air Control (IAC) system. See DIAGNOSTIC AIDS in CODE 35 chart in appropriate 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. Fuel economy is noticeably lower than it was on this vehicle at one time.

Possible Cause & Correction - Check the following

1. Check for proper operation of thermostatic air cleaner damper door. 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. Use temperature-to-resistance table in SENSOR RANGE CHARTS article.
4. Ensure speedometer is properly calibrated.
5. Check engine compression.
6. Check for dragging brakes.
7. Check A/C for "full time" operation.
8. Ensure initial ignition timing is properly set and check for proper EST and ESC operation.
9. Check for proper operation of TCC.
10. Check air pump output not shifting to catalytic converter upon signal from TVS and/or 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 CHECK in BASIC TESTING article.
12. Check oxygen sensor for silicone or lead contamination.
13. Remove spark plugs and check for wet plugs, cracks, improper gap, burned electrodes or heavy carbon deposits.
14. Check engine compression.
15. Check for dragging brakes.

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

Possible Cause & Correction Check the following

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

A mild to severe ping, usually worse under acceleration. The engine makes sharp metallic knocks that change with amount of acceleration.

Possible Cause & Correction Check the following

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 open all the time.
7. Ensure that TCC system is operating properly.
8. Ensure correct PROM is installed in ECM.
9. Remove carbon from engine with top engine cleaner.
10. If excessive carbon exists in combustion chamber, check for excessive oil burning due to leaking valve guide seals.
11. Check for incorrect basic engine parts such as camshaft, cylinder heads and pistons.
12. Check that PROM in vehicle is correct for that vehicle. Check with dealer for latest application information.

Vehicle fails emission test. Vehicle may also have excessive "rotten egg" smell (hydrogen sulfide) being emitted from exhaust pipe.

Possible Cause & Correction Check the following

1. Check for lead contamination of catalytic converter. Look for removal/tampering at restrictor in fuel filler neck.
2. Check that air is not being diverted to exhaust manifold during normal engine operation and that air 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 also has excessive odor being emitted, check all systems and components that could cause engine to run rich. See DIAGNOSTIC AIDS in CODE 45 chart in appropriate TESTS W/CODES article.
4. If emission test shows excessive oxides of nitrogen (NOx) emissions, check all systems and components that could cause engine to run lean or to run too hot. See DIAGNOSTIC AIDS in CODE 44 chart in appropriate TESTS W/CODES article.

INTERMITTENT PROBLEM DIAGNOSIS

Intermittent fault testing requires the duplication of circuit or component failure, in order to identify the fault. These procedures may lead to the computer recording a fault code (on some systems) which may help in diagnosis.

If problem vehicle does not produce fault codes, it will be necessary to monitor voltage or resistance values using a DVOM while attempting to reproduce conditions which will create an intermittent fault. A change in status on the DVOM will indicate 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 the voltmeter while performing test procedure will indicate area of fault. See TEST PROCEDURES under INTERMITTENTS in this article.

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

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