MTC Advanced Diagnostics & Troubleshooting 2 — Questions and Answers
Question 1: A four-stroke outboard engine cranks but will not fire. Spark is confirmed at all cylinders. What is the MOST logical next diagnostic step?
- Replace the ignition coils
- Verify fuel pressure and injector pulse signal (Correct answer)
- Rebuild the carburetor
- Check the starter motor current draw
Correct answer: Verify fuel pressure and injector pulse signal
When spark is confirmed, the next step is to verify fuel delivery — pressure and injector operation — before moving to mechanical checks.
Question 2: Which symptom most strongly indicates a cracked engine block in a marine inboard?
- White smoke from exhaust after warm-up
- Coolant level that drops without visible external leaks (Correct answer)
- Intermittent misfire at high RPM
- Oil pressure dropping at idle only
Correct answer: Coolant level that drops without visible external leaks
A cracked block allows coolant to migrate internally without visible puddles, causing a steady unexplained coolant loss.
Question 3: A stern-drive unit vibrates heavily at speeds above 3,000 RPM but is smooth at idle. The most likely cause is:
- A worn gimbal bearing
- A damaged or out-of-balance propeller (Correct answer)
- Low gear oil level
- A faulty trim sender
Correct answer: A damaged or out-of-balance propeller
Propeller imbalance or damage creates centrifugal vibration that intensifies with RPM, not present at low speeds.
Question 4: When diagnosing an EFI marine engine with a lean misfire fault code, the technician should first inspect:
- Injector resistance values
- Mass air flow sensor output and vacuum lines for leaks (Correct answer)
- Ignition timing advance curve
- Throttle position sensor sweep
Correct answer: Mass air flow sensor output and vacuum lines for leaks
Vacuum leaks and MAF errors both introduce unmetered air, causing a lean condition before suspecting fuel-side components.
Question 5: A marine battery bank shows 12.6 V at rest but collapses to 9.8 V under starter load. This indicates:
- A faulty battery isolator
- Sulfated or failed battery cells with high internal resistance (Correct answer)
- Undersized battery cables
- A defective voltage regulator
Correct answer: Sulfated or failed battery cells with high internal resistance
Healthy batteries maintain near-resting voltage under load; severe voltage drop under cranking load is the classic sign of internal cell failure or sulfation.
Question 6: An inboard diesel exhibits black smoke at all throttle positions. Which condition is LEAST likely to cause this symptom?
- Overfueling due to a stuck injector
- Restricted air intake or clogged air filter
- Low compression from worn rings
- Low coolant temperature from a stuck-open thermostat (Correct answer)
Correct answer: Low coolant temperature from a stuck-open thermostat
A stuck-open thermostat causes long warm-up and may affect performance but does not directly cause continuous black smoke.
Question 7: When using a vacuum gauge to diagnose an inboard gasoline engine, a needle that oscillates widely between 14 and 20 in-Hg at idle most likely indicates:
- Late ignition timing
- A stuck-open choke
- A leaking head gasket on one or more cylinders (Correct answer)
- Low oil pressure
Correct answer: A leaking head gasket on one or more cylinders
Wide, rhythmic vacuum oscillation at idle is the classic gauge signature of a leaking head gasket allowing intermittent combustion pressure into the intake manifold.
MTC Advanced Diagnostics & Troubleshooting 2
A four-stroke outboard engine cranks but will not fire.
Spark is confirmed at all cylinders.
What is the MOST logical next diagnostic step?