Incorrect Air/Fuel Mixture

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

CAUSE (BY SYSTEM/COMPONENT):

FUEL SYSTEMS
- For water contaminated fuel.
- Fuel system pressure.
- TPS for sticking or binding.
- For a 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 SYSTEMS
- 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 for wetting, cracks, wear, improper gap, burned electrodes, or heavy deposits.