GEN III Or GEN IV Engine Misfire - keywords P0300 arm cam camshaft lifter wear worn engine L33 L59 L76 LC9 LH6 LM7 LMG LR4 LS2 LS4 LQ4 LQ9 LY5 LY6 miss rocker roller rough valve

This PI was superseded to update models and model years. Please discard PIP4138B.

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

An engine misfire that may be caused by lifter roller or camshaft wear on any 4.8 5.3 or 6.0 V-8 engine. The misfire described in this PI may be difficult to diagnose, hard to feel, and can occur on any cylinder.

Complete the current diagnostics for any symptom or trouble code found.

If the vehicle is showing misfires on a single cylinder, and the SI diagnostics are inconclusive, there may be a valve train, or camshaft related concern.

A complete compression test, injector balance test, and a cylinder leakage test must be completed and documented. If these diagnostics are inconclusive, continue with the information found in this PI.

Valve train concerns that are causing difficult to diagnose conditions are listed below. The tests and/or inspections should be completed by the easiest to find to the most difficult.

1. Valve carbon build up from poor fuels causing valve sticking/valve seating concerns.
2. Broken Valve Springs
3. Valve sticking due to tight valve to guide clearances.
4. Valve seat leaking due to valve face to seat contact.
5. Lifter lock pin not locking (only on the Active Fuel Management (AFM) equipped engines).
6. Lifter roller pitting or flat spotting.
7. Lifter roller damage and/or cam lobe wear.

Thoughts on root causing some of the misfire concerns that may be encountered.

1. The use of Top Tier fuels, and GM fuel system treatment plus, may alleviate many of the valve carbon deposit concerns. The use of Upper Engine and Fuel Injector Cleaner diluted with gasoline may be introduced into the throttle body as an induction cleaner. Mixing instructions from bulletin number 03-06-04-030 can be used. Extreme care must be taken not to hydrolock the engine.
2. Valve to guide clearance concerns may be temperature related. To diagnose this concern the technician can use SI procedures for On Vehicle Valve Stem Oil Seal and Valve Spring Replacement. When the spring has been removed from the valve, wrap a rubber band tightly around the valve stem near the tip of the valve. (This step will ensure valve cannot be dropped into the combustion chamber). Release the air pressure in the cylinder that is holding the valve closed. Work the valve up and down in the guide while turning the valve a complete 360 degrees. Any binding felt in the valve would suggest a tight or damaged valve guide. Repair valve guide concerns as needed.
3. Valve seating/sealing concerns should be found with cylinder leakage testing. If a valve face or valve seat concern has been found, repair or replace only the effected cylinder head and valves.
4. If a lifter lock pin concern has been found and the rocker arm is not being actuated correctly, replace only that bank of lifters. (all eight lifters left or right side) Be sure to rotate the lifter roller and look for any damage. Camshaft inspection should also be completed when the lifter has been removed.
5. If a lifter has flat spots or pitting the camshaft and all lifters will need to be replaced.

If a camshaft or lifter concern has been verified, complete the current SI procedures to repair the vehicle. Engineering has collected samples of returned parts and is investigating camshaft/lifter concerns. This PI will be updated when new information becomes available.

Note: When diagnosing a misfire on a GEN III or GEN IV engine, and a cylinder head has been removed, replacement of that bank of lifters (if only one head is removed replace only those 8 lifters) is suggested.

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.