ASE A1 Engine RepairGeneral Engine DiagnosisHard
A customer complains of poor engine performance and excessive exhaust smoke. A technician performs an oil analysis and finds high levels of fuel dilution in the engine oil. Which of the following is the MOST likely cause of this condition?
- AContaminated engine air filter
- BOverfilled engine oil
- CStuck-open fuel injector
- DLeaking oil cooler
Show answer & explanationAnswer & explanation
Correct answer: C. Stuck-open fuel injector
Fuel dilution in engine oil means that raw fuel is mixing with the lubricating oil. A stuck-open fuel injector would continuously spray fuel into the cylinder, much of which would not burn and would instead wash down the cylinder walls, past the piston rings, and into the crankcase, leading to significant fuel dilution of the engine oil. This also explains poor engine performance and excessive exhaust smoke (often black due to rich mixture).
Why the other options are wrong
- A. A contaminated air filter would restrict airflow, leading to a rich condition and potentially black smoke, but it wouldn't directly cause fuel to dilute the engine oil unless the rich condition was so severe it led to repeated misfires and fuel washing down the cylinders.
- B. Overfilled engine oil would cause aeration and potentially foaming, but not fuel dilution.
- D. A leaking oil cooler would typically cause coolant to mix with oil (milky oil), not fuel.
Fuel Dilution in Oil
The presence of unburnt fuel in the engine's lubricating oil, reducing oil viscosity and lubricating properties.
- Causes: stuck-open fuel injectors, excessive short trips, prolonged idling, rich fuel mixture problems.
- Symptoms: reduced oil viscosity, strong fuel smell from oil, increased oil level, engine wear.
- Diagnosed via oil analysis or by addressing fuel system issues.
Memory trick: Oil's bad mix, clues in the fix.