With Computer Controls

IDLE SPEED PROBLEMS

Description
The engine will not idle properly. The idle speed is either too fast, too slow, erratic/surging, or the engine stalls at idle.

Explanation
Idle speed problems are generally not the result of a misadjusted idle speed screw. Idle speed problems may usually be traced back to an Air/Fuel Mixture problem or an ignition problem. Idle speed problems may also be caused by problems with the Idle speed control system or the emissions system.

NOTE: Other systems such as Air Conditioning or Power Steering may affect idle speed control. When diagnosing idle speed problems, always take note of any systems or components which may be loading the engine.

Possible Causes

Incorrect Air/Fuel Mixture

Vacuum Leaks
At idle, engine vacuum is high and intake airflow is low. Air which enters the intake through a vacuum leak will have a greater affect on the engine at idle than at higher speeds.

Small vacuum leaks tend to increase idle speed (as the computer compensates with additonal fuel) and may richen the Air/Fuel Mixture. Large vacuum leaks may result in a lean air/fuel mixture (the computer is unable to compensate) and a lower idle speed.

Testing
- Inspect all vacuum lines, hoses, and gaskets for leakage. With the engine running, gently squeeze each hose at its vacuum source and check for a change in idle speed. Diagrams
- Inspect the brake booster hose for leakage. With the engine running, gently squeeze the hose at its source and check for a change in idle speed.

Ignition Problems

Incorrect Timing
Ignition Timing and Idle Speed closely affect each other. When adjustments are made to either one, the other should always be checked.

Retarded timing will tend to lower idle speed while advanced timing will tend to raise idle speed.

Testing
- Check Ignition Timing and adjust if necessary.

Worn or Fouled Spark Plugs
Partially fouled spark plugs may operate satisfactory at high speeds but at idle the engine may begin to miss and run poorly.

Testing
- Remove spark plugs and check for wear and fouling, replace if necessary.
- Check spark plug gap, and adjust if necessary.

Bad Ignition Cables
Ignition Cables may develop high resistance or may chafe against metal parts. The chafing wears away the insulation, allowing the ignition cable to short out.

Testing
- Visually inspect all ignition cables for chafing or evidence of arcing. Verify that all cables are routed properly.

WARNING: Do not inspect ignition cables while the engine is running.

- Inspect all terminals for corrosion, clean if necessary.
- Check Ignition Cables for proper resistance.

Idle Speed Control

TPS Sensor
A throttle position sensors which is out of calibration may fool the Fuel Control system into thinking the throttle plates are open when the engine is at idle (with the throttle plates open, idle speed control is turned off). This may result in erratic idle speeds.

Testing
- Check Computers and Control System for Diagnostic Trouble Codes (DTC). Testing and Inspection
- Visually check the Throttle Position Sensor for damage or tampering.

Misadjusted Minimum Airflow
Minimum Airflow adjustments or Idle Speed stop screws which are adjusted improperly may result in poor idle speed control.

Adjustments which are opened excessively may admit too much air for the idle speed control system to compensate for. It may also hold the throttle plates open too far, fooling the Fuel Control system into thinking the engine is not at idle.

Testing
- Check the Minimum Airflow adjustments or Idle Speed stop screws for obvious signs of tampering or misadjustment, adjust if necessary.

Emissions

Defective PCV Valve
Idle speed control calibration assumes that a specific amount of air enters the intake through the PCV valve. A PCV valve which is stuck shut or plugged may result in a lower idle speed.

Testing
- Remove and inspect the PCV valve, replace if necessary. Positive Crankcase Ventilation

EGR Valve Stuck Open
An EGR valve which is stuck open or is not fully seated will admit exhaust gas to the intake while at idle. This results in stalling or a low and unstable idle speed.

Testing
- Remove the EGR valve and verify the seat is fully shut, replace if necessary. Exhaust Gas Recirculation
- Cycle the EGR valve several times and verify the valve is not binding.