MST Marine Engine Diagnostics & Repair 3 — Questions and Answers
Question 1: A marine technician performs a compression test on a 4-cylinder engine and finds cylinders 2 and 3 have equally low readings. What is the most likely cause?
- Worn piston rings in both cylinders
- Blown head gasket between cylinders 2 and 3 (Correct answer)
- Stuck open intake valves on both cylinders
- Incorrect valve timing
Correct answer: Blown head gasket between cylinders 2 and 3
Adjacent cylinders with equally low compression indicate a blown head gasket between those cylinders, allowing combustion pressure to leak between them.
Question 2: What is the correct procedure for checking injector spray pattern on a port-fuel injected marine engine?
- Run the engine and observe exhaust color
- Remove injectors and test on a bench injector tester (Correct answer)
- Measure fuel pressure at the rail with engine running
- Perform a cylinder contribution test with a scan tool
Correct answer: Remove injectors and test on a bench injector tester
Bench testing removed injectors on an injector tester allows direct visual inspection of spray pattern, atomization quality, and leak-down rate.
Question 3: An inboard marine engine vibrates severely at all RPMs. The engine mounts appear good. What should the technician inspect next?
- Carburetor jetting
- Propeller for damage or imbalance (Correct answer)
- Distributor cap and rotor
- Engine oil viscosity
Correct answer: Propeller for damage or imbalance
A damaged, bent, or imbalanced propeller is a leading cause of whole-RPM-range vibration in inboard installations after engine mounts are verified good.
Question 4: When rebuilding a marine engine, bearing clearances are checked using Plastigage. The measured clearance is 0.001 inches but the specification is 0.0015–0.003 inches. What should the technician do?
- Use the bearing as-is since it is close to spec
- Install a thinner bearing insert to increase clearance (Correct answer)
- Hone the bearing journal to increase clearance
- Replace the crankshaft
Correct answer: Install a thinner bearing insert to increase clearance
A clearance below specification creates insufficient oil film; installing a thinner bearing insert increases the running clearance to within the specified range.
Question 5: A marine diesel engine cranks normally but will not start in cold weather. The glow plug indicator light functions properly. What should the technician check first?
- Fuel injector spray pattern
- Individual glow plug resistance with an ohmmeter (Correct answer)
- Compression ratio
- Fuel filter restriction
Correct answer: Individual glow plug resistance with an ohmmeter
A functioning glow plug light only indicates the circuit is energized; measuring individual glow plug resistance identifies failed elements that won't heat the pre-combustion chamber.
Question 6: Which condition would cause a marine engine's oil pressure gauge to read high at idle but drop to normal at higher RPMs?
- Worn main bearings
- Stuck closed oil pressure relief valve (Correct answer)
- Blocked oil filter bypass
- Low oil level
Correct answer: Stuck closed oil pressure relief valve
A stuck closed oil pressure relief valve prevents excess pressure from being bypassed, causing artificially high pressure at idle that can normalize as pump output increases with RPM.
Question 7: A technician is diagnosing a fuel-injected outboard that runs rough and has a lean misfire code. Fuel pressure is within spec. What is the next logical diagnostic step?
- Replace all fuel injectors
- Check for vacuum leaks in the intake manifold (Correct answer)
- Increase fuel pressure regulator setting
- Replace the oxygen sensor
Correct answer: Check for vacuum leaks in the intake manifold
Lean misfire with correct fuel pressure suggests unmetered air entering the intake; vacuum leaks allow air to bypass the throttle and MAF sensor, causing a lean condition.
MST Marine Engine Diagnostics & Repair 3
A marine technician performs a compression test on a 4-cylinder engine and finds cylinders 2 and 3 have equally low readings.
What is the most likely cause?