ASE Engine Repair A1 — Questions and Answers
Question 1: What is the primary function of the camshaft in an internal combustion engine?
- To control the timing and operation of intake and exhaust valves (Correct answer)
- To pressurize and circulate engine oil
- To regulate coolant flow through the engine block
- To convert the reciprocating motion of pistons into rotational force
Correct answer: To control the timing and operation of intake and exhaust valves
The camshaft controls the opening and closing of the intake and exhaust valves in sync with piston movement. It is driven by the crankshaft via a timing belt, chain, or gears at half the crankshaft speed in a 4-stroke engine.
Question 2: A technician measures excessive crankshaft end play during an engine rebuild. What is the MOST likely cause?
- Worn or damaged thrust bearings (Correct answer)
- Excessive main bearing clearance
- A warped crankshaft flange
- Incorrect rod bearing crush
Correct answer: Worn or damaged thrust bearings
Crankshaft end play is controlled by thrust bearings (thrust washers). Worn or damaged thrust bearings allow excessive axial movement of the crankshaft. If end play exceeds specification, the thrust bearings must be replaced.
Question 3: Which condition would MOST likely cause low compression in one cylinder while all others test normal?
- A burned or bent valve (Correct answer)
- Worn piston rings across the engine
- Incorrect ignition timing
- A clogged fuel injector
Correct answer: A burned or bent valve
A burned or bent valve prevents the combustion chamber from sealing properly during the compression stroke, resulting in low compression in that cylinder only. Worn piston rings affect multiple cylinders more evenly, while a cracked block or head gasket failure typically affects adjacent cylinders.
Question 4: When performing a cylinder leak-down test, air escaping from the oil filler cap indicates leakage past which component?
- Worn or damaged piston rings (Correct answer)
- A leaking intake valve
- A failed head gasket
- A leaking exhaust valve
Correct answer: Worn or damaged piston rings
Air escaping from the oil filler cap during a leak-down test indicates combustion gases are leaking past the piston rings into the crankcase (blow-by). Air at the throttle body or air filter suggests intake valve leakage; air at the exhaust pipe suggests exhaust valve leakage; air at the radiator suggests a head gasket leak.
Question 5: What is the purpose of measuring main and rod bearing clearance with Plastigage during an engine rebuild?
- To verify the oil film clearance between the bearing and crankshaft journal (Correct answer)
- To measure the bearing crush after cap installation
- To check the hardness of the bearing material
- To confirm the crankshaft is within roundness specifications
Correct answer: To verify the oil film clearance between the bearing and crankshaft journal
Plastigage is a soft plastic thread placed between the bearing and journal before torquing the cap. After disassembly, the width of the crushed Plastigage is compared to a scale to determine oil clearance. Correct clearance ensures an adequate hydrodynamic oil film that prevents metal-to-metal contact and premature bearing wear.
Question 6: An engine exhibits excessive oil consumption with blue smoke from the exhaust that is WORST on deceleration. What is the MOST likely cause?
- Worn or damaged valve stem seals (Correct answer)
- Worn piston rings and cylinder walls
- A cracked piston crown
- Excessive positive crankcase ventilation (PCV) flow
Correct answer: Worn or damaged valve stem seals
Worn or damaged valve stem seals allow engine oil to be drawn past the valve stems and into the combustion chamber under the high intake manifold vacuum present during deceleration. This produces blue smoke that is most noticeable on deceleration. Worn piston rings typically produce blue smoke under acceleration or load rather than deceleration.
ASE Engine Repair A1
What is the primary function of the camshaft in an internal combustion engine?