MTC Engine Performance & Tuning 3 — Questions and Answers
Question 1: When performing a cylinder leak-down test on a marine engine, what does a reading of 25% leakage indicate?
- Excellent cylinder sealing
- Normal wear within spec
- Excessive leakage requiring investigation (Correct answer)
- The test is inconclusive
Correct answer: Excessive leakage requiring investigation
Leak-down readings above 20% generally indicate excessive leakage from worn rings, bad valves, or head gasket failure requiring further diagnosis.
Question 2: What is the correct procedure for setting valve clearance on an overhead cam marine engine?
- Set clearance with the engine at operating temperature only
- Set clearance with the cam lobe pointing away from the follower (Correct answer)
- Set clearance with the cam lobe pointing directly at the follower
- Set clearance only when cylinder is on overlap
Correct answer: Set clearance with the cam lobe pointing away from the follower
Valve clearance must be set when the cam lobe points away from the follower (valve fully closed) so the clearance is measured at the base circle.
Question 3: A two-stroke marine engine produces excessive power loss. Air-cooled reeds are found worn and not sealing. What is the likely result of this condition?
- Increased fuel efficiency
- Loss of crankcase compression and poor scavenging (Correct answer)
- Excessive oil consumption
- Overheating of the cylinder head
Correct answer: Loss of crankcase compression and poor scavenging
Worn reed valves allow air/fuel mixture to escape back into the intake, reducing crankcase pressure and scavenging efficiency, causing significant power loss.
Question 4: Which of the following best describes volumetric efficiency in a marine engine?
- The ratio of actual air/fuel mixture drawn in vs. theoretical cylinder volume (Correct answer)
- The ratio of brake horsepower to indicated horsepower
- The efficiency of fuel combustion in the cylinder
- The ratio of torque to engine displacement
Correct answer: The ratio of actual air/fuel mixture drawn in vs. theoretical cylinder volume
Volumetric efficiency is the ratio of actual air volume inducted into the cylinder compared to the theoretical displacement volume, expressed as a percentage.
Question 5: On a marine engine with electronic fuel injection, a MAP sensor reading lower than actual manifold pressure would cause the ECM to:
- Enrich the fuel mixture
- Lean the fuel mixture (Correct answer)
- Advance ignition timing
- Increase idle speed
Correct answer: Lean the fuel mixture
If the MAP sensor reads lower pressure than actual, the ECM calculates less air entering the engine and commands less fuel, causing a lean condition.
Question 6: What is the primary function of a harmonic balancer on a marine engine crankshaft?
- Balance the weight of the flywheel
- Dampen torsional vibration in the crankshaft (Correct answer)
- Drive the camshaft timing belt
- Provide a mounting point for the timing marks
Correct answer: Dampen torsional vibration in the crankshaft
A harmonic balancer absorbs and dampens torsional vibration (twisting forces) generated by combustion pulses in the crankshaft to prevent fatigue failure.
Question 7: A marine technician observes that engine oil pressure drops significantly at operating temperature but is normal when cold. The MOST likely cause is:
- Faulty oil pressure sending unit
- Worn main and rod bearings (Correct answer)
- Clogged oil filter
- Defective oil pump relief valve stuck closed
Correct answer: Worn main and rod bearings
Worn bearing clearances allow increased oil flow and pressure loss; hot oil is thinner, making the pressure drop more pronounced at operating temperature.
MTC Engine Performance & Tuning 3
When performing a cylinder leak-down test on a marine engine, what does a reading of 25% leakage indicate?