P0300 DTC And Engine Misfires At Idle - Possible Leaking Intake Valves - keywords CEL compression controls excessive high leakage leakdown low MIL miss

This PI was superseded to request cylinder leakage rates on the repair order, remove outdated parts information, and highlight that this concern occurs only at idle. Please discard PIP4013.

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

On rare occasions, a SES light may be encountered due to a P0300 DTC. Typically, the P0300 is due to a single cylinder at idle that may or may not be felt and disappears off of idle. This may be the result of a leaking intake valve.

Important: The engine misfires described in this PI, which occur as a result of the leaking intake valves mentioned above, will occur at idle. If the engine misfires do not occur at idle, this PI does not apply.

If the SI diagnostics do not isolate a cause for this concern, perform a cylinder leakage test as outlined in the "Cylinder Leakage Test" procedure in SI and document all leakage rates on the repair order. When performing the cylinder leakage test, it is very important to hold the crankshaft with the related piston at top dead center to ensure that the valves are fully closed or inaccurate results may be obtained. If an aftermarket leakdown tester is used, the instructions of the aftermarket tester should also be referenced.

If a leaking intake valve is found, follow the updated cylinder head replacement procedure in 06-06-01-017C and replace the cylinder head casting using all new valves and seals.

Important: The updated cylinder head replacement procedure outlined in 06-06-01-017C no longer requires removal of the oil pan, front cover, water pump, or crankshaft dampener. Before replacing a cylinder head by following 06-06-01-017C, also review PIP3940A, which contains some tips for using the lower timing gear tensioner holding tool (EN48464) that is mentioned in 06-06-01-017C.

The replacement cylinder head does not come with any valvetrain components so it will be necessary to perform the following steps before installing the new cylinder head. Please refer to the "Cylinder Head Disassemble" and Cylinder Head Assemble" procedures in SI to perform the following suggestions:

1. Install all new valves and valve seals on the exhaust and intake side using the part numbers listed in the GMSPO catalog.
2. It is not necessary to lap the new valves, however, the valve to seat contact can be confirmed by lightly lapping the valves with fine or extra fine lapping compound. A light lap is defined as, no measurable amount of seat material is removed, but the witness marks of the contact area can be seen.
3. Inspect all other components from the original cylinder head. If there is no problem found with any of the components, transfer them to the new cylinder head (including the original valve springs). Place all of the remaining components in their original locations on the new cylinder head.

Please follow this diagnostic or repair 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.