310S Engine Systems and Repair 2 — Questions and Answers
Question 1: What is the purpose of the oil clearance between a crankshaft journal and its bearing?
- To allow thermal expansion of the crankshaft
- To create a hydrodynamic oil wedge for lubrication (Correct answer)
- To reduce the weight of the rotating assembly
- To allow for manufacturing tolerances only
Correct 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.
Question 2: A technician finds milky residue on the oil filler cap. What is the most likely cause?
- Overfilled engine oil
- Coolant mixing with engine oil
- Use of incorrect oil viscosity
- Normal condensation from short-trip driving (Correct answer)
Correct 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.
Question 3: What is the correct procedure for measuring cylinder bore out-of-round?
- Measure at the top and bottom of the bore with a tape measure
- Use a telescoping gauge or bore gauge at thrust and non-thrust sides at the same height (Correct answer)
- Measure the piston diameter and subtract from the nominal bore size
- Use a feeler gauge between the piston and cylinder wall
Correct 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.
Question 4: What causes a timing belt to skip teeth?
- Excessive oil contamination on the belt
- A worn or improperly tensioned timing belt tensioner (Correct answer)
- Using premium fuel instead of regular
- A faulty mass airflow sensor
Correct 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.
Question 5: What is the purpose of crosshatching the cylinder walls during an engine rebuild?
- To increase the bore diameter to specification
- To provide oil retention for proper ring seating (Correct answer)
- To remove rust and corrosion from the cylinder walls
- To create a smooth surface for reduced friction
Correct 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.
Question 6: An engine with variable valve timing (VVT) has a rattling noise at startup. What is the most likely cause?
- Worn main bearings
- A failed VVT phaser or actuator with internal wear (Correct answer)
- Loose heat shields on the exhaust manifold
- Incorrect spark plug gap
Correct 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.
310S Engine Systems and Repair 2
What is the purpose of the oil clearance between a crankshaft journal and its bearing?