Without 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 the result of a problem with an 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 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.

Vacuum leaks will lean out the Air/Fuel Mixture and result in a low, rough 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.

Emissions

Defective PCV Valve
Base Idle Speed is calculated assuming that a specific amount of air enters the intake through the PCV valve. A PCV valve which is stuck open may act like a vacuum leak.

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.