High HC, Low or Normal CO - at Idle
Symptom: High Hydrocarbons, normal or low Carbon Monoxide at idle.
POSSIBLE CAUSE ACTION
Electrical misfire ^ Inspect ignition system.
SEE IGNITION SYSTEM/ OSCILLOSCOPE PATTERNS.
Incorrect spark advance ^ Check spark controls.
SEE IGNITION SYSTEM/ TIMING SPECIFICATIONS
Lean Air-Fuel mixture ^ Check fuel pressure.
^ Check O2 sensor.
^ Perform injector balance test (SFI only).
Vacuum leak ^ Inspect vacuum hoses for proper routing and damaged lines.
^ Refer to Vehicle Emission Control Information (VECI) label under hood.
^ Check intake manifold area.
Leaking exhaust valves or piston rings ^ Perform compression test and cylinder leak down test.
Idle speed too low ^ Perform Minimum Airflow Test.
SEE COMPUTERS AND CONTROL SYSTEMS/ DIAGNOSIS AND TESTING.
EGR Valve open at idle ^ Check EGR system.