Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing an engine compression test on a vehicle that exhibits a significant loss of power and rough running. The dry compression test results for all cylinders are between 70-80 psi. What is the most likely cause?
- AA clogged catalytic converter.
- BWorn piston rings or cylinder walls.
- CIncorrect valve timing.
- DA restricted exhaust system.
Show answer & explanationAnswer & explanation
Correct answer: B. Worn piston rings or cylinder walls.
Consistently low compression across all cylinders, especially in the 70-80 psi range, strongly indicates general wear of the piston rings or cylinder walls. While other issues can cause power loss, widespread low compression points to internal engine wear.
Why the other options are wrong
- A. A clogged catalytic converter would restrict exhaust flow and cause power loss, but it would not directly cause consistently low engine compression readings.
- C. Incorrect valve timing would typically result in low compression, but a 'dry' test alone doesn't differentiate it from ring wear as definitively as a 'wet' test.
- D. A restricted exhaust system would cause power loss but not necessarily such low and consistent compression readings across all cylinders.
Low Compression All Cylinders
When all engine cylinders show significantly lower than specified compression readings, indicating widespread internal engine wear or a general mechanical issue affecting all cylinders.
- Suggests worn piston rings, cylinder walls, or possibly incorrect valve timing.
- Often accompanied by rough running and power loss.
- Requires further diagnosis, such as a wet compression test to differentiate ring wear.
Memory trick: Compression tells the story, low means wear or timing.