Perform Idle Speed Drop Test


Connect tachometer to measure engine RPM

With car running:

  1. Loosen injector lines at injectors one at a time.
  2. Observe amount of change or no change in idle speed to determine “weak” or “dead” cylinder(s).
    • Idle speed should drop approximately the same on all cylinders as injector lines are loosened.
  3. If the problem cannot be isolated to a particular cylinder see Diagnosis II.
  4. If a “weak” or “dead” cylinder is found



  • Wear safety goggles when performing idle speed drop test.
  • After perfoming idle speed drop test, be sure to wipe off all spilled fuel with rubbing alcohol. Diesel fuel spilled on hoses will cause the hoses to deteriorate.
  • Keep a fire extinguisher accessible.