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.
See ADJUSTMENT PROCEDURES in EMISSIONS.



Air-Fuel mixture too lean Adjust air/fuel ratio.
See ADJUSTMENT PROCEDURES in EMISSIONS.



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



EGR at idle Check EGR controls. See EXHAUST GAS RE-CIRCULATION
SYSTEM in TESTING PROCEDURES.



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



Idle speed too low Check and adjust the idle speed.
See ADJUSTMENT PROCEDURES in EMISSIONS.
Check the operation of any idle speed
control devices.