Rough, Unstable, or Incorrect Idle, Stalling

Definition: The engine runs unevenly at idle. If severe, the vehicle may shake. Also, the idle speed may vary (called "hunting"). Either condition may be severe enough to cause stalling. Engine idles at incorrect speed.

Perform careful visual and physical check as described at the beginning of Symptoms.

Perform "Diagnostic Circuit Check."

^ CHECK
- MAP sensor. Use CHART C1-D. Chart C-1D Manifold Absolute Pressure (MAP) Output Check
- Throttle for sticking shaft or binding linkage. This will cause a high TPS voltage (open throttle indication) and the ECM will not control idle. TPS voltage should be less than 1.25 volts with throttle closed.
- Battery cables and ground straps for poor contact. Erratic voltage will cause the IAC valve to change its position, resulting in poor idle quality.
- Ignition wires for shorts or faulty insulation.
- Ignition system for moisture, dust, cracks, burns, etc. Spray plug wires with fine water mist to check for shorts.
- For possibility of misfiring, crossfiring, or cutting out under load or at idle. If present, refer to the "Ignition System Check" Chart. Chart C-4C Ignition System Check (Remote Coil)
- Secondary ignition voltage output with J 26792 (ST-125) spark tester or equivalent.
- For worn distributor shaft.
- For moisture in distributor cap.
- Ignition pick-up coil resistance and connection.
- Ignition coil connections.
- ECM and ignition system for faulty grounds.
- Proper operation of EST. See "Ignition Systems."
- Spark plugs. Look for wear, wetness, cracks, improper gap, burned electrodes, and heavy deposits.
- Fuel system for restricted filter or improper pressure. Use CHART A-7. Chart A-7 Fuel System Pressure Test
- Injectors for leakage. Pressurize system by energizing fuel pump through the underhood fuel pump test connector.
- EGR operation. Use Code 32 chart. Powertrain Controls - ECM/PCM
- For vacuum leaks at intake manifold gasket.
- Idle Air Control system. Use Code 35 chart. Powertrain Controls - ECM/PCM
- Electrical system voltage. IAC valve will not move if voltage is below 9 volts or greater than 17.8 volts.
- PCV valves for proper operation by placing finger over inlet hole in each valve end several times. Valve should snap back. If not, replace valve. Ensure that valve is correct part. Also check PCV hoses.
- Canister purge system for proper operation.
- PROM for correct application (Consult Service Bulletins.)
- Throttle shaft or TPS for sticking or binding. TPS voltage should read less than 1.25 volts on a "Scan" tool with the throttle closed.
- MAP sensor output. Use CHART C1-D and/or check sensor by comparing it to the output on a similar vehicle if possible. Chart C-1D Manifold Absolute Pressure (MAP) Output Check
- Oxygen sensor for silicone contamination from contaminated fuel or use of improper RTV sealant. The sensor will have a white, powdery coating and will cause a high but false signal voltage (rich exhaust indication). The ECM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
- Coolant sensor for a shift in calibration. Use Code 14 or Code 15 chart. Powertrain Controls - ECM/PCM Powertrain Controls - ECM/PCM
- A/C refrigerant pressure for high pressure. Check for overcharging or faulty pressure switch.
- P/N switch circuit on vehicle with automatic transmission. Use CHART C-1A. Chart C-1A Park/Neutral Switch Diagnosis
- Generator output voltage. Repair if less than 9 volts or more than 16 volts.
- Engine valve timing and compression. Refer to Engine. Engine
- For worn or incorrect basic engine parts such as cam, heads, pistons, etc. Also check for bent pushrods, worn rocker arms, and broken or weak valve springs. Refer to Engine. Engine
- Check ignition timing. Refer to underhood emission label for proper procedure.
- For the possibility of an exhaust restriction or improper valve timing, perform the following test.
1. With engine at normal operating temperature, connect a vacuum gauge to any convenient vacuum port on intake manifold.
2. Run engine at 1000 rpm and record vacuum reading.
3. Increase engine speed slowly to 2500 rpm. Note vacuum reading at steady 2500 rpm.
4. If vacuum at 2500 rpm decreases more than 3" Hg from reading at 1000 rpm, the exhaust system should be inspected for restrictions.
5. Disconnect exhaust pipe from engine and repeat Steps 3 & 4. If vacuum still drops more than 3" Hg with exhaust disconnected, check valve timing.
- For overheating and possible causes. Look for the following.
- Low or incorrect coolant solution. It should be a 50/50 mix of GM #1052753 anti-freeze coolant (or equivalent) and water.
- Loose accessory drive belt.
- Restricted air flow to radiator, or restricted water flow through radiator.
- Faulty or incorrect thermostat.
- Faulty fan clutch.
^ If the system is running RICH, (block learn less than 118), refer to "Diagnostic Aids" of Code 45. Powertrain Controls - ECM/PCM
^ If the system is running LEAN, (block learn greater than 138), refer to "Diagnostic Aids" of Code 44. Powertrain Controls - ECM/PCM
^ CHECK
- For possibility of misfiring, crossfiring, or cutting under load or at idle. Locate misfiring cylinder(s) by performing the following test.
1. Start engine. Disconnect idle air control valve. Remove one spark plug wire from a spark plug and ground it against the engine.
2. Note drop in engine speed.
3. Repeat for all six cylinders.
4. Stop engine and reconnect idle air control valve.
If the engine speed dropped equally (within 50 rpm) on all cylinders, proceed through the causes listed. If there was no drop or excessive variation in engine speed on one or more cylinders, check for spark on the respective cylinder(s) with J 26792 (ST-125) spark tester or equivalent. If there is no spark, refer to IGNITION SYSTEM. Ignition System
If spark is present, refer to spark plug check.