Incorrect Air/Fuel Mixture

Symptom: Engine runs normally but the air/fuel mixture is either rich or lean.

POSSIBLE CAUSES
Fuel System
- Water contaminated fuel.
- Fuel system pressure.
- Throttle Position Sensor (TPS) sticking or binding.
- Faulty in-tank fuel pump check valve, which would allow the fuel in the lines to drain back to the tank after the engine is stopped.

Emission Systems
- EGR operation. Valve should be fully seated closed with engine "OFF".

Ignition System
- Proper sparking voltage and duration.
- Worn distributor shaft.
- Bare and shorted wires.
- Improper pickup coil resistance or poor connections.
- Loose ignition coil connections.
- Moisture in distributor cap.
- Spark plugs wet, cracked, worn, improper gap, burned electrodes, or heavy deposits.