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 / Testing and Inspection / Oscilloscope Patterns and Waveforms.
Incorrect initial timing ^ Check base timing.
SEE Ignition System / Adjustments.
^ Check CAM timing.
Incorrect spark advance ^ Check spark controls.
SEE Ignition System / 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/Testing and Inspection/Procedures.
EGR Valve open at idle ^ Check EGR system.
Exhaust Gas Recirculation