During a dry compression test, which fault would cause low compression readings to significantly rise during a wet compression test?

During a dry compression test, which fault would cause low compression readings to significantly rise during a wet compression test?

• A. Burnt valves
• B. Defective head gasket
• C. Worn piston rings
• D. Worn valve guides

Answer: (C)

In a compression test, you’re distinguishing where the leakage is coming from—rings sealing versus valves or the head gasket. If a dry test shows low compression and the reading rises significantly when you squirt oil into the cylinder, it means the oil temporarily seals gaps around worn rings and reduces leakage past the rings. That’s why the fault is worn piston rings—the oil helps the rings seal better in the cylinder, restoring compression. The other faults involve leakage through the valve seats or the head gasket, and adding oil doesn’t fix those leaks, so the compression wouldn’t improve much.

In a compression test, you’re distinguishing where the leakage is coming from—rings sealing versus valves or the head gasket. If a dry test shows low compression and the reading rises significantly when you squirt oil into the cylinder, it means the oil temporarily seals gaps around worn rings and reduces leakage past the rings. That’s why the fault is worn piston rings—the oil helps the rings seal better in the cylinder, restoring compression. The other faults involve leakage through the valve seats or the head gasket, and adding oil doesn’t fix those leaks, so the compression wouldn’t improve much.