Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing a cylinder leakage test on cylinder #2. With the piston at Top Dead Center (TDC) on the compression stroke, 100 psi of shop air is applied to the cylinder. Air is heard escaping from the oil filler cap. What does this indicate?
- AA blown head gasket to another cylinder.
- BA leaking intake valve.
- CA leaking exhaust valve.
- DWorn piston rings.
Show answer & explanationAnswer & explanation
Correct answer: D. Worn piston rings.
Air escaping from the oil filler cap during a cylinder leakage test indicates that the compressed air is bypassing the piston and entering the crankcase, which is a clear sign of worn or damaged piston rings.
Why the other options are wrong
- A. A blown head gasket to an adjacent cylinder would cause air to be heard escaping from the spark plug hole of the adjacent cylinder, or bubbles in the radiator if leaking into the coolant.
- B. A leaking intake valve would cause air to be heard escaping from the throttle body or intake manifold.
- C. A leaking exhaust valve would cause air to be heard escaping from the tailpipe or exhaust manifold.
Cylinder Leakage Test - Oil Filler Cap
During a cylinder leakage test, air escaping from the oil filler cap indicates leakage past the piston rings into the crankcase.
- Confirms worn or damaged piston rings.
- Requires the piston to be at TDC compression.
- Distinguishes ring wear from valve or head gasket leaks.
Memory trick: Oil Cap's Hiss, Rings are Remiss!