Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A technician is diagnosing a vehicle that consistently consumes 1.5 quarts of oil every 500 miles, but there is no visible blue smoke from the exhaust, even during varied driving conditions. There are also no external oil leaks evident. What is the most likely cause of this oil consumption?
- AInternal head gasket leak into the combustion chamber.
- BA clogged PCV valve.
- CWorn valve stem seals.
- DOil dilution by fuel.
Show answer & explanationAnswer & explanation
Correct answer: B. A clogged PCV valve.
Significant oil consumption without visible blue smoke or external leaks often points to issues with the PCV system. A clogged PCV valve can lead to excessive crankcase pressure, forcing oil past seals and rings without necessarily burning it in a way that produces visible smoke, or it can cause oil to be drawn into the intake manifold in liquid form.
Why the other options are wrong
- A. An internal head gasket leak consuming oil into the combustion chamber would produce blue smoke.
- C. Worn valve stem seals typically produce visible blue smoke, especially on acceleration after deceleration or idle.
- D. Oil dilution by fuel would increase the oil level and thin it, not cause a decrease in oil level (consumption).
Oil Consumption w/o Blue Smoke/Leaks
Diagnosing engine oil consumption when traditional indicators like blue smoke or external leaks are absent, often suggesting crankcase ventilation issues.
- Clogged PCV valve can cause oil consumption without blue smoke.
- Excessive crankcase pressure forces oil past seals.
- Oil can be drawn into intake as liquid, burning less visibly.
Memory trick: No smoke, no drip, yet oil takes a trip.