High HC, Low or Normal CO -- at Idle

Condition at idle: Abnormally high hydrocarbon emissions while carbon monoxide content remains normal or low.

POSSIBLE CAUSE ACTION


Electrical misfire Inspect ignition system



Ignition timing too far advanced. Check ignition timing.




Air-Fuel mixture too lean Check for unmeasured air entering intake system. Also check for low fuel pressure and/or plugged or restricted injectors.



Vacuum leak Inspect vacuum hoses for
proper routing and damaged lines
Check intake manifold area



EGR at idle



Leaking exhaust valves or piston rings Perform compression test and
cylinder leak down test



Idle speed too low Check idle speed.