Engine Systems and Repair Flashcards

What is the purpose of the oil clearance between a crankshaft journal and its bearing?

Answer: To create a hydrodynamic oil wedge for lubrication

The precise oil clearance allows pressurized oil to form a hydrodynamic wedge between the journal and bearing, preventing metal-to-metal contact during operation.

A technician finds milky residue on the oil filler cap. What is the most likely cause?

Answer: Normal condensation from short-trip driving

Milky residue only on the filler cap is commonly caused by condensation from short-trip driving where the engine never reaches full operating temperature to evaporate moisture. If coolant were mixing with oil, the dipstick oil would also appear milky.

What is the correct procedure for measuring cylinder bore out-of-round?

Answer: Use a telescoping gauge or bore gauge at thrust and non-thrust sides at the same height

Out-of-round is determined by measuring the bore diameter at the thrust side (perpendicular to the crankshaft) and non-thrust side (parallel to the crankshaft) at the same height, then comparing the readings.

What causes a timing belt to skip teeth?

Answer: A worn or improperly tensioned timing belt tensioner

A worn or improperly tensioned timing belt tensioner allows the belt to become slack, which can cause it to skip teeth on the sprockets, leading to valve timing errors and potential engine damage.

What is the purpose of crosshatching the cylinder walls during an engine rebuild?

Answer: To provide oil retention for proper ring seating

Crosshatch honing creates a pattern of fine grooves that retain oil on the cylinder walls, promoting proper piston ring seating and lubrication during the break-in period and normal operation.

An engine with variable valve timing (VVT) has a rattling noise at startup. What is the most likely cause?

Answer: A failed VVT phaser or actuator with internal wear

VVT phasers rely on oil pressure to maintain proper position. When worn internally, they rattle at startup before oil pressure builds sufficiently to control them.