Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairHard
A vehicle arrives at a shop with a complaint of a rough idle, especially when cold, and occasional misfires. There is a 'Check Engine' light illuminated, and DTC P0301 (Cylinder 1 Misfire Detected) is stored. The technician performs a cylinder balance test and confirms a misfire on cylinder 1. What is the LEAST likely cause of this specific misfire, considering the provided context?
- AA clogged fuel injector for cylinder 1
- BA faulty EVAP purge valve stuck open
- CA leaking intake manifold gasket affecting cylinder 1
- DA fouled spark plug in cylinder 1
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty EVAP purge valve stuck open
While a faulty EVAP purge valve stuck open can cause a lean condition and rough idle, it typically affects all cylinders or a bank of cylinders, not specifically one cylinder (P0301). The other options (fouled spark plug, clogged injector, leaking intake manifold gasket) are all common causes of a cylinder-specific misfire.
Why the other options are wrong
- A. A clogged fuel injector for a specific cylinder would directly cause a misfire on that cylinder due to insufficient fuel.
- C. A leaking intake manifold gasket can cause a vacuum leak that affects one or more specific cylinders, leading to a lean misfire.
- D. A fouled spark plug is a very common cause of a cylinder-specific misfire.
Cylinder-Specific Misfire Causes
Issues leading to a misfire on only one particular cylinder, often due to a problem with that cylinder's fuel, ignition, or compression.
- Common causes: bad spark plug, faulty coil, clogged injector, compression loss.
- Less likely to be system-wide issues (e.g., MAF, fuel pump, EVAP purge) which affect multiple cylinders.
- DTCs like P0301-P0308 indicate specific cylinder misfires.
Memory trick: P0301: One cylinder's problem, not the whole team!