High HC, Low or Normal CO - at Idle

Condition: High Hydrocarbons, normal or low Carbon Monoxide at idle.

POSSIBLE CAUSE ACTION


Electrical misfire Inspect ignition system.
SEE Powertrain Management/Ignition System/Testing and Inspection/Oscilloscope Patterns and Waveforms.

Incorrect spark advance Check spark controls.
SEE Ignition System/ TIMING SPECIFICATIONS

Lean Air-Fuel mixture Check fuel pressure.
Check O2 sensor.
Perform injector balance test (MPI).
.

Vacuum leak Inspect vacuum hoses for proper routing and damaged lines.
Refer to Vehicle Emission Control Information (VECI) label under hood.
Check intake manifold area.

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

Idle speed too low Perform Minimum Airflow Test.
SEE Powertrain Management/Computers and Control Systems/Testing and Inspection/Procedures.

EGR Valve open at idle Check EGR system.
Testing and Inspection