A technician is diagnosing a vehicle with a consistent misfire on multiple cylinders and black, sooty exhaust smoke. The 'Check Engine' light is on, and scan tool data shows very high negative short-term and long-term fuel trim values. What is the most likely cause?
- ALow fuel pressure due to a failing fuel pump.
- BA faulty engine coolant temperature (ECT) sensor reporting a constantly cold engine.
- CA clogged catalytic converter.
- DA large vacuum leak in the intake manifold.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty engine coolant temperature (ECT) sensor reporting a constantly cold engine.
High negative fuel trim values indicate the ECU is trying to remove a significant amount of fuel because it detects a rich condition. An ECT sensor falsely reporting a constantly cold engine would cause the ECU to continuously command a rich fuel mixture (like a 'choke'), leading to misfires, black smoke, and the ECU attempting to lean out the mixture, resulting in high negative fuel trims.
Why the other options are wrong
- A. Low fuel pressure would cause a *lean* condition or misfires due to insufficient fuel, leading to *positive* fuel trims as the ECU tries to add more fuel.
- C. A clogged catalytic converter would primarily cause restricted exhaust flow, leading to power loss and potentially overheating, but not directly cause a rich condition with black smoke and high negative fuel trims.
- D. A large vacuum leak would cause a *lean* condition, leading to *positive* fuel trims as the ECU tries to add fuel.
ECT Sensor Failure (Rich Condition)
A faulty Engine Coolant Temperature (ECT) sensor that incorrectly reports a cold engine, causing the ECU to command excessive fuel, leading to a rich condition, misfires, and black smoke.
- ECU uses ECT for fuel mixture calculations.
- False 'cold' reading leads to continuous enrichment (like a choke).
- Causes negative fuel trims as ECU tries to reduce fuel.
- Symptoms: misfires, black smoke, poor fuel economy.
Memory trick: Black smoke, negative trim? Sensor's fooled, engine's grim!