Step-by-step engine maintenance checklist

Work down the list in order. Tick each step as you complete it — the progress bar tracks the whole service.

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Shut the engine down, isolate the batteries and close the seacock before working on any component. Always follow your engine manufacturer's manual for torque figures, intervals and fluid specifications.

1. Pre-start checks

Every outing

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Check engine oil level and colourPull the dipstick, wipe, re-insert and read. Oil should sit between the marks and look amber-to-brown — milky oil means water ingress, stop and investigate.Rag, torch

Check coolant level in the header tankOnly open the cap on a cold engine. Top up with the correct antifreeze mix; a steadily dropping level points to a leak or head gasket issue.

Inspect the alternator / water pump beltPress the longest run of belt — around 10 mm deflection is right. Look for glazing, cracks and black dust on the mounts.

Open the raw water seacock and check the strainerClear weed and debris from the basket, confirm the lid seal is seated, then confirm water flow at the exhaust within 15 seconds of starting.

Check the bilge and drip tray under the engineFresh oil, coolant or fuel in the tray tells you where a leak started before it becomes a failure at sea.

2. Running checks

Each start-up

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Confirm cooling water at the exhaustNo water within 15–20 seconds: shut down immediately and check the impeller, strainer and seacock before restarting.

Watch oil pressure and temperature come upOil pressure should rise within seconds. Note normal operating temperature for your engine so you can spot creep early.

Read the exhaust smokeBlack = over-fuelling or overload. Blue = burning oil. White = water or unburnt fuel. A clean warm engine should be near-invisible.

Listen and feel for new noises or vibrationCheck engine mounts and the shaft coupling if vibration is new — soft mounts and misalignment wear out sterngear fast.

3. Monthly / 50-hour service

Every 50 engine hours

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Drain water from the primary fuel filter bowlCrack the drain screw into a jar until clean diesel runs. Water and black sludge mean it is time for a new element.Jar, spanner, rag

Check battery terminals and charge stateClean corrosion, retighten, and confirm resting voltage of ~12.7 V. Verify the alternator lifts it to 13.8–14.4 V when running.Multimeter, wire brush

Squeeze-test coolant and exhaust hosesHoses should feel firm, not spongy or crunchy. Check the exhaust elbow and hose clips for soot trails and rust weeps.

Check gearbox oil levelUse the correct oil for your box (many take ATF, not engine oil). Check for water contamination at the same time.

4. Annual / winter service

Every season or 200–250 hours

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Change engine oil and filter while warmRun to temperature, extract the old oil, fit a new filter with a smear of clean oil on the gasket, refill and re-check the level after a short run.Oil extractor, filter wrench, new filter and oil

Replace the raw water impellerFit a new gasket, count the vanes on the old impeller and retrieve any missing pieces from the heat exchanger before running up.Impeller kit, screwdriver, pliers

Replace primary and secondary fuel filter elementsBleed the system afterwards until the engine runs smoothly with no air in the lines.

Inspect and replace sacrificial anodesReplace pencil anodes in the heat exchanger and the shaft/saildrive anode once around half wasted.

Check engine alignment and mount conditionWith the boat afloat and settled, check coupling faces are parallel within 0.05 mm and mount rubbers are not collapsed.Feeler gauges, spanners

Winterise the cooling and fuel systemsFlush the raw water circuit with antifreeze, fill the tank to reduce condensation, add biocide, and fog or oil the cylinders if laying up long-term.

General guidance only — see the disclaimer.