ASE A1 Engine RepairCylinder Head and Valve TrainHard
A technician is performing a compression test on an engine. Cylinder #2 yields a reading of 90 psi, while the other cylinders are all around 150 psi. When a small amount of oil is squirted into cylinder #2 and the test is repeated, the compression reading remains at 95 psi. This result most strongly indicates which of the following problems?
- AA blown head gasket.
- BA leaking intake valve.
- CA cracked cylinder block.
- DWorn piston rings.
Show answer & explanationAnswer & explanation
Correct answer: B. A leaking intake valve.
If adding oil to the cylinder does not significantly increase compression, it indicates that the compression loss is not due to worn piston rings. Instead, it points to a leak past the valves or a head gasket. Given a single cylinder issue and no indication of coolant contamination, a leaking valve (either intake or exhaust) is highly probable, as oil would not seal a valve leak.
Why the other options are wrong
- A. A blown head gasket could cause low compression but might also show signs of coolant in the oil or oil in the coolant. More importantly, adding oil would not seal a head gasket leak.
- C. A cracked cylinder block is less common and would likely manifest with other severe symptoms like coolant leaks, but adding oil would not typically seal such a crack to affect compression significantly.
- D. Worn piston rings would typically show a significant increase in compression after adding oil, as the oil temporarily seals the rings.
Wet Compression Test
A diagnostic test where a small amount of oil is added to a cylinder after a dry compression test to differentiate between compression loss due to piston rings versus valves.
- Performed after a dry compression test.
- Oil temporarily seals worn piston rings.
- No change in compression indicates valve or head gasket issue.
Memory trick: Oil no help? Valve's the culprit, or gasket's felt.