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

Incorrect spark advance Check spark controls,

Lean Air-Fuel mixture Check Air-Fuel ratio, O2 sensor, perform injector balance test, inspect regulator operation (check vacuum line and return hose), check fuel pressure

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

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

Idle speed too low Check operation of Automatic Idle Speed (AIS) motor,

EGR Valve open at idle Check EGR system.

Restricted PCV valve and/or hose Check PCV system, Positive Crankcase Ventilation