ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is diagnosing an engine with a persistent misfire in Cylinder 4. After checking spark and fuel delivery to Cylinder 4, which appear to be adequate, a compression test reveals 60 PSI, while all other cylinders are consistently at 150 PSI. What is the MOST likely cause of the misfire in Cylinder 4?
- AWorn piston rings or damaged valve
- BRestricted exhaust manifold
- CClogged fuel injector
- DFaulty ignition coil
Show answer & explanationAnswer & explanation
Correct answer: A. Worn piston rings or damaged valve
A significantly low compression reading (60 PSI vs. 150 PSI) in a single cylinder, despite adequate spark and fuel, points directly to a mechanical issue causing a lack of cylinder sealing, such as worn piston rings, a damaged valve, or a blown head gasket.
Why the other options are wrong
- B. A restricted exhaust manifold would affect all cylinders or a bank of cylinders, causing overall power loss, but not typically a single-cylinder low compression issue.
- C. A clogged fuel injector would cause a misfire due to lack of fuel, but would not affect compression.
- D. A faulty ignition coil would cause a misfire due to lack of spark, but would not affect compression.
Low Cylinder Compression
Low cylinder compression indicates a loss of sealing within the combustion chamber, preventing the engine from effectively building pressure for combustion.
- Common causes include worn piston rings, damaged valves (burnt, bent), blown head gasket.
- Results in misfires, reduced power, rough idle, and poor fuel economy.
- Often diagnosed with a compression test or leak-down test.
Memory trick: Misfire? Check spark, fuel, then compression's power pool.