Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairEasy
A technician is diagnosing a vehicle with a persistent P0301 (Cylinder 1 Misfire) code. They have confirmed spark and fuel delivery to cylinder 1. The next step is to check for proper compression. A compression test on cylinder 1 yields a reading of 80 PSI, while the other cylinders are consistently at 150 PSI. What is the MOST likely cause of the P0301 code in this scenario?
- AAn internal engine mechanical fault.
- BA faulty ignition coil on cylinder 1.
- CA clogged fuel injector on cylinder 1.
- DExcessive carbon buildup on the intake valves.
Show answer & explanationAnswer & explanation
Correct answer: A. An internal engine mechanical fault.
A significantly lower compression reading (80 PSI vs. 150 PSI) indicates a mechanical issue within the cylinder, such as worn piston rings, a burnt valve, or a blown head gasket. This loss of compression will directly cause a misfire, even if spark and fuel are present.
Why the other options are wrong
- B. Spark has already been confirmed, so a faulty ignition coil is not the cause of the misfire.
- C. Fuel delivery has been confirmed, so a clogged fuel injector is not the cause.
- D. While excessive carbon on valves can cause issues, a 70 PSI difference in compression strongly suggests a more severe mechanical fault than just carbon buildup.
Low Compression Misfire
A significant loss of compression in a cylinder indicates an internal engine mechanical fault, which will cause a misfire despite proper ignition and fuel delivery.
- Compression difference >15-20% is significant.
- Indicates worn rings, burnt valve, head gasket failure.
- Direct cause of misfire.
- Requires mechanical repair, not ignition/fuel.
Memory trick: Spark, Fuel, then Air Compression 'Checks'.