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 ACTION

Electrical misfire Inspect ignition system



Too much spark advance or Check and adjust timing in
initial timing "Adjustment Procedures" in Emissions



Air-Fuel mixture too lean Adjust air/fuel ratio



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



EGR at idle Check EGR system in "Testing
Procedures" in Emissions



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



Idle speed too low Check and adjust idle speed in
"Adjustment Procedures" in Emissions
Check operation of any idle speed
control devices