Perform both a compression test and a cylinder leakage test. See 3.8 TROUBLESHOOTING, Compression Test and 3.8 TROUBLESHOOTING, Cylinder Leakdown Test. If further testing is needed, remove suspect head(s) and inspect for the following:
Check Prior to Cylinder Head Removal
1. Oil level too high.
2. Oil carryover.
3. Breather hose restricted.
4. Restricted oil filter.
Check After Cylinder Head Removal
1. Oil return passages for clogging.
2. Valve guide seals.
3. Valve guide to valve stem clearance.
4. Gasket surface of both head and cylinder.
5. Cylinder head casting porosity allowing oil to drain into combustion chamber.
6. O-ring damaged or missing from oil pump/crankcase junction.
7. If the above checks do not reveal the cause, remove the cylinder to inspect for excess piston ring wear. Also verify
the piston ring gaps are properly staggered.
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