ASE A1 Engine RepairLubrication and Cooling SystemsMedium
An engine exhibits abnormally high oil consumption, blue-tinted exhaust smoke, and fouled spark plugs. A compression test yields acceptable readings across all cylinders. Which of the following is the most probable cause?
- AA faulty PCV (Positive Crankcase Ventilation) system
- BA worn piston ring
- CWorn valve guides or valve stem seals
- DA leaking intake manifold gasket
Show answer & explanationAnswer & explanation
Correct answer: C. Worn valve guides or valve stem seals
High oil consumption with blue smoke and fouled plugs, combined with good compression, strongly suggests oil is entering the combustion chamber from above the piston. Worn valve guides or valve stem seals allow oil to leak past the valves into the combustion chamber.
Why the other options are wrong
- A. A faulty PCV system can cause oil consumption or pressure issues, but typically doesn't manifest with such clear blue smoke and fouled plugs while compression remains good.
- B. Worn piston rings would typically result in low compression readings, in addition to oil consumption and smoke.
- D. A leaking intake manifold gasket would usually cause a vacuum leak, rough idle, or coolant/oil mixing, but not directly lead to oil burning with blue smoke and good compression.
Valve Stem Seal Failure
Failed valve stem seals allow engine oil to leak past the valve stems into the combustion chamber, leading to oil burning, blue exhaust smoke, and fouled spark plugs, often without affecting engine compression.
- Oil consumption and blue smoke on startup or deceleration
- Fouled spark plugs
- Compression usually remains good
Memory trick: Blue smoke, good squeeze? Seals are the disease!