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 initial timing ^ Check base timing.
SEE IGNITION SYSTEM/ ADJUSTMENT PROCEDURES.

^ Check CAM timing.


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 Powertrain Management/Computers and Control Systems/Testing and Inspection/Procedures.


EGR Valve open at idle ^ Check EGR system.
Exhaust Gas Recirculation