ASE Engine Diagnostics 4 — Questions and Answers
Question 1: Oil pressure is normal on a cold engine but drops very low at warm idle. What is the most likely cause?
- Worn crankshaft main and rod bearings (Correct answer)
- Clogged oil pickup screen
- Stuck-closed pressure relief valve
- Oil viscosity too high
Correct answer: Worn crankshaft main and rod bearings
Excessive bearing clearance lets thin, hot oil escape, lowering pressure at idle.
Question 2: A deep knock is loudest under load and goes away when the spark plug wire on one cylinder is disconnected. What is the likely cause?
- Worn main bearing
- Excessive crankshaft end play
- Worn connecting rod bearing (Correct answer)
- Loose flywheel
Correct answer: Worn connecting rod bearing
Rod bearing knock lessens or stops when that cylinder's combustion load is removed.
Question 3: A vehicle puffs blue smoke for a few seconds at startup after sitting overnight, then clears. What is the most likely cause?
- Worn piston rings
- Leaking head gasket
- Clogged PCV valve
- Worn valve stem seals (Correct answer)
Correct answer: Worn valve stem seals
Oil seeps past worn valve seals into the cylinders while the engine sits, then burns off at startup.
Question 4: Which test confirms that combustion gases are entering the cooling system?
- Chemical block test (combustion leak tester) (Correct answer)
- Cooling system pressure test with the engine off
- Thermostat opening test
- Radiator flow test
Correct answer: Chemical block test (combustion leak tester)
The block test fluid changes color when exposed to exhaust gases in the coolant.
Question 5: A light ticking noise occurs at half crankshaft speed. Which component is most likely the source?
- Piston pin
- Valvetrain component such as a lifter (Correct answer)
- Connecting rod bearing
- Main bearing
Correct answer: Valvetrain component such as a lifter
Valvetrain parts run at camshaft speed, which is half the crankshaft speed.
Question 6: An engine gives off heavy black exhaust smoke. What does this usually indicate?
- Burning oil
- Burning coolant
- Rich air-fuel mixture (Correct answer)
- Lean air-fuel mixture
Correct answer: Rich air-fuel mixture
Black smoke is carbon from unburned fuel caused by a rich mixture.
Question 7: The engine oil looks milky and light brown on the dipstick. What is the most likely cause?
- Overfilled crankcase
- Fuel dilution from leaking injectors
- Excessive blow-by from worn rings
- Coolant leaking into the oil (Correct answer)
Correct answer: Coolant leaking into the oil
Coolant mixing with oil produces a milky, light brown appearance, often from a head or intake gasket failure.
ASE Engine Diagnostics 4
Oil pressure is normal on a cold engine but drops very low at warm idle.
What is the most likely cause?