Rough Unstable Idle

Symptom: The engine runs unevenly. If bad enough, the car may shake. The engine idle may hunt or fluctuate in RPM and may be bad enough to stall.

POSSIBLE CAUSE ACTION


PCV valve malfunction ^ Check PCV system.


Incorrect Air-Fuel mixture ^ Check fuel pressure.
^ Check O2 sensor.

^ Perform injector balance test (SFI only).


EGR malfunction ^ Check EGR system.
Exhaust Gas Recirculation


Faulty EVAP Canister purge ^ Check Fuel Evaporative System.
Evaporative Emissions System


Restricted exhaust system ^ Check Catalytic Converter.
Catalytic Converter


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


Dirty air filters ^ Clean or replace


Poor or contaminated fuel ^ Recommend higher grade fuel