Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A vehicle owner complains of a noticeable increase in fuel consumption and a slight loss of power, but the engine runs smoothly and has no obvious noises or vibrations. A dry compression test yields readings of 160 psi, 165 psi, 158 psi, and 162 psi for cylinders 1 through 4, respectively. What is the most likely conclusion based on these compression readings?
- AThere is a significant vacuum leak causing the power loss.
- BThere is a severe internal engine problem requiring overhaul.
- CThe catalytic converter is likely restricted.
- DThe engine's internal sealing components (rings, valves, head gasket) are in good condition.
Show answer & explanationAnswer & explanation
Correct answer: D. The engine's internal sealing components (rings, valves, head gasket) are in good condition.
Compression readings between 160-165 psi and within 10-15% of each other (in this case, all are very close) indicate that the engine's internal sealing components are in good condition. The reported symptoms are likely due to other issues, not internal mechanical wear.
Why the other options are wrong
- A. While a vacuum leak can cause power loss and poor fuel economy, these compression readings rule out internal engine issues as the primary cause, which is what the compression test measures.
- B. These compression readings are well within a healthy range, indicating no severe internal engine problem.
- C. A restricted catalytic converter would cause power loss and poor fuel economy, but it would not directly affect compression readings in this manner.
Normal Engine Compression
Engine compression readings that are within the manufacturer's specifications and show minimal variation (typically less than 10-15%) between cylinders.
- Indicates good sealing of piston rings, valves, and head gasket.
- Suggests the engine's internal mechanical condition is healthy.
- If symptoms exist, diagnosis should focus on external systems (fuel, ignition, exhaust, vacuum).
Memory trick: Good compression, good engine heart.