Worn or loose rocker arms.

The rocker arm bearing end caps and/or needle bearings should be intact within the rocker arm assembly.

Replace the valve rocker arms, as required. Refer to Valve Rocker Arm and Push Rod Replacement.

Worn or bent pushrods.

Replace the pushrods, as required. Refer to Valve Rocker Arm and Push Rod Replacement.

Stuck valves.

Carbon buildup on the valve stem can cause the valve not to close correctly.

Repair or replace, as required. Refer to Cylinder Head Disassemble.

A worn or mis-aligned timing chain above acceptable tolerances.

Replace the timing chain and sprockets, as required. Refer to Camshaft Timing Chain and Sprocket Replacement.

Worn camshaft lobes

Replace the camshaft and valve lifters. Refer to Camshaft Replacement.

Sticking lifters

Replace the camshaft and valve lifters. Refer to Camshaft Replacement.