Rough Idle; OK Above Idle

Idle problems


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.

Diagnosis Procedure

  1. 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.
  2. 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.
  3. Check for fuel leaks
    • Fuel leaks at fuel lines and fuel delivery valves reduce fuel pressure affecting fuel delivery — a rough idle can result.
  4. Check injection pump mounting brackets
    • Loose pump mounting brackets can cause pump misalignment and injection timing change resulting in idle problems.
  5. 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.
  6. Test Injectors
    • If a ‘weak’ or ‘dead’ cylinder is found, the injector for that cylinder should be checked with this procedure.
  7. Check / adjust valve clearance
  8. Check compression


Quality Check

  1. With fully warm engine, raise engine speed to 3000 RPM for about 5 seconds.
  2. Release the throttle and let the engine return to idle.
  3. 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