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 initial timing Check and adjust timing in
"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 (if equipped) in "Testing
Procedures" in Emissions



Leaking exhaust valves or piston rings Perform compression test and
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