Gen III Or Gen IV V-8 Engine Misfire - keywords P0300 arm engine L33 L59 LH6 LM7 LR4 LS2 LQ4 LQ9 lifter misfire miss rocker rough valve

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

Engine misfire 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.

General Motors Brand Quality and Engineering is interested in any V-8 engine with a cam lobe or lifter concerns. This would include any excessive wear to the lifter roller, or the cam lobe itself.

A complete compression test, an 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 diagnostics are listed below, and 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. Valve sticking due to tight valve to guide clearances.
3. Valve seat leaking due to valve face to seat contact.
4. Lifter lock pin not locking (only on the Active Fuel Management (AFM) equipped engines).
5. Lifter roller pitting or flat spotting.
6. 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 guides 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 cam will need to be replaced.

In the event a flat spot or pitting is discovered on the lifter or cam, please contact Brand Quality Manager Jay Dankovich to discuss the repairs, and parts returns.

Contact can either be made by E-mail or phone call.

Jay Dankovich

[email protected]

586-947-8881

When contacting Jay Dankovich the root cause should have already been found. Engineering is looking only for cam lobe or lifter concerns.

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.