A technician is diagnosing a vehicle with a complaint of poor fuel economy and a Check Engine Light (CEL) illuminated. Using a scan tool, the technician observes a consistently high short-term fuel trim (STFT) and long-term fuel trim (LTFT) values, both above +20%. The upstream oxygen (O2) sensor voltage fluctuates between 0.1V and 0.9V, but the downstream O2 sensor voltage is consistently around 0.7V. Which of the following is the most likely cause?
- AA restricted fuel return line.
- BA clogged catalytic converter.
- CA faulty Engine Coolant Temperature (ECT) sensor reporting a cold engine.
- DA leaking exhaust manifold gasket.
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking exhaust manifold gasket.
High positive fuel trims indicate the engine control unit (ECU) is adding more fuel than commanded, suggesting a lean condition. A leaking exhaust manifold gasket allows unmetered air into the exhaust stream, making the upstream O2 sensor report a lean condition, causing the ECU to enrich the mixture, leading to high positive fuel trims. The downstream O2 sensor reading indicates the catalytic converter is still working efficiently.
Why the other options are wrong
- A. A restricted fuel return line would increase fuel pressure, causing a rich condition and negative fuel trims, not positive.
- B. A clogged catalytic converter would primarily affect exhaust flow and often cause power loss, but wouldn't directly cause high positive fuel trims in this manner. It would also likely cause the downstream O2 sensor to mirror the upstream.
- C. A faulty ECT sensor reporting a cold engine would cause the ECU to richen the mixture, leading to negative fuel trims, not positive.
Exhaust Leak & Fuel Trims
An exhaust leak before the upstream oxygen sensor introduces unmetered air, causing the sensor to report a lean condition, which the ECU compensates for by increasing fuel delivery, leading to high positive fuel trims.
- Exhaust leaks before upstream O2 sensor trick the ECU.
- ECU adds fuel to correct perceived lean condition.
- Results in high positive short-term and long-term fuel trims.
Memory trick: Fuel trims are 'positive' when the engine 'adds' fuel, trying to 'fix' a lean condition.