Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairMedium
A client complains their vehicle has poor fuel economy and occasionally emits black smoke from the exhaust, especially during acceleration. A technician observes that the upstream oxygen (O2) sensor voltage remains consistently high (above 0.8V) during closed-loop operation. Which of the following is the MOST likely cause?
- AA leaking fuel injector
- BA vacuum leak in the intake manifold
- CA restricted exhaust system
- DA faulty Engine Coolant Temperature (ECT) sensor indicating a cold engine
Show answer & explanationAnswer & explanation
Correct answer: A. A leaking fuel injector
A consistently high upstream O2 sensor voltage (above 0.8V) indicates a rich fuel condition. Black smoke and poor fuel economy further confirm excessive fuel. A leaking fuel injector would continuously deliver too much fuel into a cylinder, leading to this rich condition.
Why the other options are wrong
- B. A vacuum leak typically causes a lean condition (excess air), which would result in a low O2 sensor voltage.
- C. A restricted exhaust primarily causes a loss of power and elevated exhaust back pressure, not directly a rich condition indicated by high O2 sensor voltage.
- D. An ECT sensor indicating a cold engine would cause the PCM to enrich the mixture, but a 'consistently high' O2 voltage in closed-loop operation points more directly to a component causing continuous over-fueling, like a leaking injector, rather than a sensor signaling for enrichment.
Oxygen (O2) Sensor Voltage Interpretation (Upstream)
The upstream O2 sensor measures oxygen content in the exhaust to determine air-fuel ratio. High voltage (0.8-1.0V) indicates a rich mixture, while low voltage (0.1-0.2V) indicates a lean mixture.
- Crucial for closed-loop fuel control.
- Switches rapidly between high and low voltage in a healthy system.
- Consistently high voltage = rich; consistently low voltage = lean.
Memory trick: High voltage, rich mix, black smoke. Low voltage, lean mix, white smoke.