ASE A1 Engine RepairGeneral Engine DiagnosisHard
A vehicle exhibits symptoms of overheating, including high coolant temperature gauge readings and coolant loss. The cooling system has been pressure tested and holds pressure. The thermostat has been replaced, and the radiator fan operates correctly. Upon inspection, the engine oil appears milky, and there are bubbles in the coolant reservoir. Which of the following is the MOST likely cause?
- AFaulty water pump
- BClogged radiator
- CStuck-open thermostat
- DFailed head gasket
Show answer & explanationAnswer & explanation
Correct answer: D. Failed head gasket
Milky engine oil indicates coolant mixing with oil, and bubbles in the coolant reservoir (especially with exhaust gas present) indicate combustion gases leaking into the cooling system. These are classic symptoms of a failed head gasket, which allows communication between the combustion chambers, oil passages, and coolant passages.
Why the other options are wrong
- A. A faulty water pump would cause overheating but not milky oil or exhaust bubbles in the coolant.
- B. A clogged radiator would cause overheating and potentially coolant loss, but not milky oil or exhaust bubbles in the coolant.
- C. A stuck-open thermostat would cause the engine to run too cool or take a long time to warm up, not overheat, have milky oil, or bubbles in the coolant.
Failed Head Gasket
Damage to the seal between the engine block and cylinder head, leading to internal leakage between combustion, oil, and coolant passages.
- Symptoms: overheating, coolant loss, milky oil, white exhaust smoke, bubbles in coolant, engine misfires.
- Can be diagnosed with a block test (for exhaust gases in coolant) or compression/leak-down test.
- Requires significant labor for repair, often involving cylinder head removal.
Memory trick: Overheating's a plight, check oil and coolant, day and night.