6.6 LB7 Knock or Tapping Noise from Engine

When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

Tapping or knock noise from engine. Possible smoke from the exhaust.

Inspect for a stuck open fuel injector.

Pull the ignition 1 relay which powers the Fuel Injection Control Module (FICM). Remove the glow plug from the suspect cylinder. Crank the engine and see if fuel comes out of the glow plug hole. If yes, replace the fuel injector. A stuck open injector may also have misfire and/or smoke.

For knocking noises or injector problems which occur off idle or during high speeds, reference SI number 796346 Fuel Injector Balance Test with a Tech 2. Individual cylinders may be canceled while driving. If a knock noise is eliminated when canceling a particular fuel injector, suspect that injector as the problem.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.