Engine Repair and Diagnosis Flashcards

An engine has normal oil pressure when cold but very low pressure at hot idle. What is the most likely cause?

Answer: Excessive main and rod bearing clearance

Worn bearings leak more oil once it thins at operating temperature, dropping hot idle pressure.

Which tool is commonly used to measure crankshaft bearing oil clearance during engine assembly?

Answer: Plastigage

Plastigage is crushed between bearing and journal, and its width indicates clearance.

An engine puffs blue smoke on startup and after deceleration from high speed, but not under steady cruise. What is the most likely cause?

Answer: Worn valve guides or valve stem seals

High manifold vacuum on deceleration and oil draining after shutdown pull oil past worn guides or seals.

Engine oil on the dipstick appears milky and light brown. What is the most likely cause?

Answer: Coolant contamination from a head gasket, cracked head, or oil cooler

Milky oil indicates coolant mixing with oil through an internal leak.

A deep knock gets louder under load and becomes quieter when the spark plug on one cylinder is disabled. What is the likely cause?

Answer: Loose rod bearing on that cylinder

Removing combustion load from the cylinder unloads its connecting rod bearing, reducing the knock.

A hollow knocking noise is loudest on a cold engine and fades as the engine warms. What is the most likely cause?

Answer: Piston slap

Excess piston-to-wall clearance shrinks as the piston expands with heat, quieting the slap.

A ticking noise occurs at one-half crankshaft speed. Which system is the most likely source?

Answer: Valvetrain

The camshaft and valvetrain turn at half crankshaft speed, so noises there occur at that rate.