Step by step
How-to guides
24 direct answers to the questions owners actually type, each one a complete answer on its own page with the steps, the tools and the point where the job stops being yours. Marine Diesel Engines Australia sells nothing and recommends nobody.
Cooling4
How do I tell if my raw-water impeller is failing?
Watch exhaust-water volume, not just presence. A failing impeller shows as reduced flow, a rising coolant temperature under load, a squealing pump, or black rubber fragments in the strainer. Inspect the impeller directly; if blades are missing, find every one before running the engine again.
20 minutes to inspectHow do I check raw-water flow on a marine diesel?
Start the engine and watch the exhaust outlet within the first ten to fifteen seconds. You want a steady, substantial pulse of water with the exhaust gas, not a trickle. If flow is weak, work backwards through the circuit: seacock, strainer, hose, impeller, heat exchanger, then exhaust elbow.
6 steps · 5 minutesHow do I clean a marine heat exchanger?
Cooling down, close the seacock, drain the raw-water side, remove the end caps and withdraw or rod out the tube stack. Clear debris mechanically, then descale with a product approved for the metals involved. Renew the end-cap O-rings, refit, and check for leaks under load.
7 steps · Half a dayHow do I stop my engine overheating under load?
First establish whether the engine reaches its rated maximum rpm. If it cannot, the boat is overloaded and heat is a symptom, not the fault. If it can, the cooling system has run out of margin: check raw-water volume, heat exchanger condition, exhaust restriction, thermostat and coolant level.
1 hour to diagnoseFuel4
How do I bleed air from a marine diesel fuel system?
Work from the tank forwards. Fill the filters with clean fuel, open the first bleed point, operate the lift pump until fuel runs free of bubbles, close it, and repeat at each successive point. Many modern common-rail engines self-prime and must never be bled at the high-pressure side.
6 steps · 20–45 minutesHow do I change marine diesel fuel filters?
Close the fuel valve, place absorbent pads, then change the primary filter and water separator first and the secondary last. Fill new filters with clean fuel before fitting, seal them by hand to the specified tightness, bleed the system, then run and re-check every joint for leaks.
8 steps · 45–90 minutesHow do I get water out of a diesel tank?
Draw from the lowest point of the tank — a drain, sump or dip tube — into a clear container and remove fluid until only clean fuel comes out. Then find the entry path, usually a deck fill seal, a breather or condensation. Heavy contamination needs professional fuel polishing or a tank clean.
4 steps · 1–3 hoursHow do I tell if I have diesel bug?
Look in the primary filter bowl and at the bottom of the tank. Diesel bug shows as dark brown or black slime, stringy deposits, or a sludge layer at the fuel-water interface, usually with a sour smell. Repeat filter blockages with no other explanation are the strongest practical sign.
30 minutesService3
How do I change the oil on a marine diesel?
Warm the engine enough to make the oil flow, stop it and isolate the start circuit. Extract or drain the oil through the designed point, remove the filter without spilling it, confirm the old gasket came away, fit the new filter as instructed, refill slightly under capacity, run briefly, then set the level exactly.
10 steps · 1 hourHow do I know when my engine is due for a service?
Use your engine's manual and act on whichever limit arrives first — running hours or calendar time. A low-hour engine still needs its annual service, because oil, coolant, impellers, belts and anodes degrade on a calendar regardless of use. Record hours and dates so the next interval is never a guess.
4 steps · 30 minutes to set upHow do I keep marine engine service records properly?
Record hours, date, what was done, the exact part numbers used, the fluids and grades, any test readings, and what you found — not just what you replaced. Findings are what make the next fault diagnosable, and a documented history is one of the few things that measurably supports a boat's value at sale.
Ten minutes per serviceElectrical3
How do I test a marine starting battery?
Measure resting voltage after several hours off charge, then measure again while cranking. A healthy 12 V battery rests near 12.6 V and should not collapse below roughly 9.6 V under cranking. A large drop with good resting voltage points to a failing battery; no drop with no crank points to connections or the starter circuit.
30 minutesHow do I find a voltage drop on a boat?
Measure voltage across each connection while the circuit is under load, not at rest. Put one probe either side of a joint, crimp or switch: a healthy connection shows a very small drop, while a corroded one shows a significant fraction of system voltage. Work along the circuit until you find the joint that is eating it.
4 steps · 45 minutesHow do I tell if my alternator is charging properly?
With the engine running above idle, measure voltage at the battery. It should be meaningfully higher than the resting voltage and within the range your regulator and battery chemistry specify. If it sits at battery voltage, the alternator is not contributing; if it climbs far above specification, the regulator needs attention immediately.
20 minutesDrives2
How do I check saildrive oil for water?
Draw a sample from the drive's designated point and inspect it against clean oil. Water shows as a milky or grey emulsion, a separated layer at the bottom, or a level that has risen. Any of those means a seal has failed and the drive needs attention before further use.
20 minutesHow do I choose the right anode for my boat?
Match the alloy to the water you keep the boat in — different alloys are specified for full seawater, brackish and fresh water — and match the anode type to what the maker of your drive or engine specifies. Then read the wear rate: anodes that vanish in weeks indicate a stray-current or bonding problem, not an anode problem.
Checked at each haul-outOperating4
How do I tell if my propeller is overloading the engine?
Run the engine to full throttle in clear water and note the maximum rpm it reaches. If it cannot reach the manufacturer's rated maximum, the propeller is overloading it. Overload shows as black smoke, higher temperatures, poor fuel economy and accelerated wear, and is corrected by propeller pitch, hull cleaning or both.
30 minutes underwayHow do I measure my boat's fuel consumption?
Log fuel added against hours run and the rpm you held, across several trips, and divide. Alternatively run a measured passage at a steady rpm and refill to the same point. Published fuel curves are a starting point only — your hull, propeller, fouling and load decide the real figure.
Several tripsHow do I respond to an engine alarm at sea?
Keep control of the boat first. Identify which alarm is sounding, reduce load, and check gauges and exhaust-water flow. Stop the engine when the alarm indicates oil pressure or overheating and stopping is safe. Sail, anchor or call for assistance early rather than late, and record the readings.
8 steps · Learn before you need itHow do I lay up a marine diesel for the season?
Change the oil and filter before the lay-up, not after, so the engine does not sit in contaminated oil. Fill the fuel tank to limit condensation, check coolant condition and concentration, protect the raw-water circuit as your manual specifies, and keep batteries maintained rather than simply disconnected.
6 steps · Half a dayBuying4
How do I choose a marine mechanic?
Ask whether the person doing the work is trained on your engine family or subcontracting it, what the quote includes, which parts are needed and whether they are held locally, who commissions the work afterwards, and what happens to the price if more is found. Compare quotes on scope, never on total.
5 steps · An afternoon of callsHow do I assess a used marine engine before buying a boat?
Insist on a genuine cold start, then run the engine under load rather than at the dock. Review the service records, inspect oil and coolant for cross-contamination, examine the exhaust and raw-water circuit for corrosion, and assess the drive separately. Age matters far less than documented maintenance and installation quality.
A full inspection, plus a sea trialHow do I decide whether to repower or repair?
Gather evidence before quotes: mechanical condition, corrosion in the cooling and exhaust path, parts availability for your engine, and the state of the installation. Then compare the repair against the full repower cost including beds, exhaust, controls, wiring and commissioning — because those systems, not the engine, dominate the bill.
Weeks, not daysHow do I size a marine generator?
Work out two numbers first: your peak AC load, including motor starting surges, and your daily energy consumption. Size the generator to the real load rather than the largest figure in the brochure — a genset that runs lightly loaded for hours will foul. Log a few typical days aboard before buying anything.
A week of logging