A vehicle experiences intermittent misfires and hesitation under acceleration. Diagnostic trouble code P0301 (Cylinder 1 Misfire Detected) is stored. The technician observes that the misfire counter for cylinder 1 increases rapidly only when the engine is warm. All other cylinders show zero misfires. What sensor input is MOST likely contributing to this condition?
- AThrottle Position (TP) sensor
- BManifold Absolute Pressure (MAP) sensor
- CVehicle Speed Sensor (VSS)
- DEngine Coolant Temperature (ECT) sensor
Show answer & explanationAnswer & explanation
Correct answer: D. Engine Coolant Temperature (ECT) sensor
The key here is the misfire occurring *only when the engine is warm*. The Engine Coolant Temperature (ECT) sensor provides crucial input to the PCM for fuel enrichment (cold start) and ignition timing adjustments. If the ECT sensor is faulty and reporting an incorrect (e.g., colder than actual) temperature when the engine is warm, the PCM might continue to provide excessive fuel enrichment for cylinder 1 (or affect its timing negatively in a warm state), leading to a rich misfire that manifests only when the engine reaches operating temperature. Other sensors would typically affect all cylinders or a different operational range.
Why the other options are wrong
- A. A TP sensor issue would affect engine response to throttle input, potentially causing hesitation or misfires across all cylinders, but not specifically a warm-engine misfire on one cylinder.
- B. A MAP sensor issue would typically affect all cylinders and cause drivability problems across various load conditions, not just a warm engine misfire on a single cylinder.
- C. The VSS primarily affects transmission shifting, cruise control, and overall vehicle speed-dependent functions. It would not directly cause a single-cylinder misfire when warm.
ECT Sensor & Fuel Trim
The Engine Coolant Temperature (ECT) sensor provides engine temperature data to the PCM, which uses it to adjust fuel delivery and ignition timing.
- Crucial for cold start enrichment and warm-up cycles.
- Incorrect readings can lead to rich or lean conditions, affecting drivability.
- Faults can cause misfires, poor fuel economy, and emissions issues.
Memory trick: When 'ONE' cylinder misfires 'WARM', check the 'ECT' and 'FUEL' for the charm.