High HC, Low or Normal CO at Idle

This condition, at idle, indicates abnormally high Hydrocarbons while the Carbon Monoxide content remains normal or low.

POSSIBLE CAUSE ACTION

Electrical misfire Inspect ignition system



Too much spark advance or initial timing Check and adjust timing



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



EGR at idle Check EGR system



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



Idle speed too low Check and adjust idle speed Check operation of any idle speed control devices