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 Check Timing and adjust
or initial timing



Air-Fuel mixture too lean Perform O2 sensor test refer to
OXYGEN SENSOR TESTING/ DIAGNOSIS
TESTING AND INSPECTION PROCEDURE
in COMPUTERS AND CONTROL SYSTEMS



Vacuum leak Check - Vacuum hoses for
proper routing and damaged lines
- Intake manifold area and TBI



EGR at idle Check EGR system.



Leaking exhaust valves Perform - Compression test
or piston rings - Cylinder leak down test



Idle speed too low Check - Controlled idle speed
- Operation of all idle speed
control devices