Duramax Diesel Pilot to Main Injection Fuel Knock Noise - keywords 6.6 6.6L accelerate engine injector LB7 LLY RPM transition

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI.

The customer concern may be a fuel knock type noise heard at 2500 to 2600 RPM, or when the vehicle is accelerated with a throttle angle above 80%.

The following is an explanation of the fuel knock noise. The Duramax Diesel has what is referred to as pilot fuel injection at lower engine speeds. Pilot fuel injection is defined as a preinjection of a small quantity of fuel, five to twenty crankshaft degrees, before the main fuel injection. This preinjection softens the main injection pressure rise. When the main injection is softened, normal diesel engine combustion noises are less noticeable. At engine speeds above 2600 RPM and a fuel quantity supply greater than 35 cubic millimeters, it is necessary to eliminate pilot injection to meet emission requirements. When driving at or above this engine RPM range, main injection combustion noise may be noticeable. It is normal and not harmful to the engine. When traveling in fifth gear the pilot injection to main injection transition is typically in the 136 kilometers per hour, or 85 miles per hour range. However, it may also be heard under heavy acceleration with throttle angle at or above 80%, when pulling loads up a hill, and during shifts at or above 2600 RPM. Customers that utilize the fifth gear inhibit switch may encounter the pilot injection to main injection transition noise at speeds around 107 kilometers per hour, or 67 miles per hour.

This sound is considered normal and no repairs should be attempted.

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.