High HC, Low or Normal CO -- at Idle

This condition, at idle, is indicated by abnormally high hydrocarbon emissions while carbon monoxide content remains normal or low.

POSSIBLE CAUSE
Electrical misfire.

CHECK
Inspect ignition system.



POSSIBLE CAUSE
Too much spark advance or initial timing.

CHECK
Ignition timing.



POSSIBLE CAUSE
Air-Fuel mixture too lean.

CHECK
Air/fuel ratio.



POSSIBLE CAUSE
Vacuum leak.

CHECK
Vacuum hoses for proper routing and damaged lines.
Intake manifold area.



POSSIBLE CAUSE
Leaking exhaust valves or piston rings.

CHECK
Perform compression test and cylinder leak down test.



POSSIBLE CAUSE
Idle speed too low.

CHECK
Idle speed.
Operation of idle speed control devices.



POSSIBLE CAUSE
EGR ON at idle.

CHECK
Check EGR system.