Idle Speed Problems

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, Power Steering, or rear window defoggers may affect idle speed control. When diagnosing idle speed problems, always take note 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. Unmetered air which enters the intake through a vacuum leak will have a greater affect on the engine at idle than at higher speeds.

Vacuum leaks tend to increase idle speed and may richen the Air/Fuel Mixture. The Fuel control system adjusts for the lowered vacuum reading by adding more fuel.

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.

Leaking Fuel Injectors
A fuel injector which leaks past the pintle valve will result in a rich mixture for the associated cylinder and lean mixture for the remaining cylinders (the computer compensates for the rich mixture by leaning out the remaining cylinders).

Testing
- Install a Fuel Pressure gauge and check Fuel Pressure.
- Start the engine and allow Fuel Pressure to stabilize.
- Turn off the engine and observer fuel pressure, if the pressure begins to drop, squeeze a section of the return line hose. If the pressure continues to drop, the Fuel Injector may be leaking. See Fuel Deliver and Air Induction for further testing. Fuel Delivery and Air Induction
- Perform a injector balance test.

Incorrect Fuel Pressure
Low or high Fuel Pressure at idle may affect the Air/Fuel Mixture sufficiently to cause an idle speed problem.

Testing
- Install a Fuel Pressure gauge and check Fuel Pressure. See Fuel Pressure Regulator for further testing. Fuel Pressure Regulator

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

Throttle Body Sludge
Oil contamination and sludge from the PCV breather hose tends to build up in the throttle body just in front of the throttle plates. This build up blocks the airflow around the throttle plates and may cause reduced or surging idle speeds.

Testing
- Check for related Technical Service Bulletins (TSB).
- Inspect the throttle body and clean as necessary.

NOTE: Some vehicles are equipped with "Sludge Tolerant" throttle bodies and should not be cleaned. See Fuel Delivery and Air Induction for further information. Fuel Delivery and Air Induction

TPS Sensor
Throttle Position Sensors which are 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. A PCV valve which is stuck open or leaking may result in a higher 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.