High HC, Low or Normal CO at Idle

DEFINITION: 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.




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



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