Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A client reports that their vehicle's engine runs rough, has reduced power, and occasionally stalls. A technician performs a compression test, and all cylinders show readings around 90-100 psi, which is significantly lower than the manufacturer's specification of 150 psi. What is the most likely general cause?
- AIncorrect valve timing
- BWorn piston rings
- CRestricted exhaust system
- DBad head gasket
Show answer & explanationAnswer & explanation
Correct answer: A. Incorrect valve timing
Low compression across all cylinders, especially if relatively even, often indicates a general engine timing issue like an incorrectly set timing belt/chain, which affects valve opening and closing, leading to poor cylinder sealing.
Why the other options are wrong
- B. Worn piston rings would typically cause varied low compression readings, and a wet compression test would show significant improvement.
- C. A restricted exhaust system would primarily cause a lack of power and rough running but would not directly cause low compression readings.
- D. A bad head gasket usually results in low compression only in affected cylinders, or adjacent cylinders, and often includes coolant consumption or contamination.
Low Compression All Cylinders
When all engine cylinders exhibit significantly lower compression readings than specified, indicating a widespread issue affecting the entire engine's sealing capability.
- Often points to a general engine timing problem.
- Could also be severely worn camshaft lobes or lifters.
- Rarely caused by uniform wear of piston rings or valves across all cylinders simultaneously.
Memory trick: Low compression everywhere? Check the engine's time and rhythm.