Warm engine has rough idle but performs OK above idle.
Rough idle can cause vibration. If a customer describes a “vibration” symptom, first asses whether or not the vibration is due to a rough idle.
- Adjust idle speed
- Idle speed should be checked and adjusted to within specifications. If the idle speed is too low, a rough idle as well as a vibration may result.
- Check for air leaks
- Loose connections or cracked hoses between the fuel tank and injection pump may result in air in the fuel system. Air trapped in injectors or fuel lines may result in a rough idle.
- Check for fuel leaks
- Fuel leaks at fuel lines and fuel delivery valves reduce fuel pressure affecting fuel delivery — a rough idle can result.
- Check injection pump mounting brackets
- Loose pump mounting brackets can cause pump misalignment and injection timing change resulting in idle problems.
- Perform idle speed drop test
- If fuel is not supplied equally by all injectors, a rough idle may result. The idle speed drop test will verify equal fuel delivery by injectors.
- Test Injectors
- If a ‘weak’ or ‘dead’ cylinder is found, the injector for that cylinder should be checked with this procedure.
- Check / adjust valve clearance
- Check compression
- With fully warm engine, raise engine speed to 3000 RPM for about 5 seconds.
- Release the throttle and let the engine return to idle.
- Measure idle speed with a tachometer and check that:
- The idle speed is correct
- The idle speed does not vary more than 50 RPM
- The idle is not rough
If test passed, return car to customer
If test failed, perform diagnosis procedure II