How to check a marine engine before buying

Insist on a cold start, take a sample of the oil and the gearbox oil, sea trial to full throttle, and pay for a compression or leak-down test and an oil analysis before you make an offer. A general boat survey usually covers the engine superficially. On a boat where the engine is a large share of the value, that is where the money is lost.

The one thing you must insist on

Start it cold. A seller who has warmed the engine before you arrive has removed the single most informative event of the inspection.

Arrange to be there at the start, or reschedule. This is not an unreasonable request and a seller’s reaction to it tells you something on its own.

The sequence

1. Ask for service records before you travel. Continuous, dated invoices from identifiable workshops are the strongest signal available. Gaps are questions.
2. Record hours and hours per year. Very low annual hours over many years is a specific risk, not a benefit.
3. Start cold. Note how long it cranks, what smoke appears and for how long, any unusual noise as it settles, and whether water flows from the exhaust within seconds.
4. Check the engine oil: level, colour, whether it smells of fuel, whether it is milky.
5. Check the coolant: level, colour, oil film on the surface, and the condition of the cap and its seat.
6. Check the gearbox oil separately. Milky means water — often a failed cooler.
7. Inspect the raw water side: strainer, impeller pump for past leaks, heat exchanger end caps, and the mixing elbow. Ask when the heat exchanger was last cleaned and when the pencil anode was last replaced. “I don’t know” is common and expensive.
8. Look for evidence of past overheating: discoloured paint, scale trails, one area of new hoses, a new heat exchanger on an otherwise original engine.
9. Check mounts, belts, hoses, clamps and the wiring loom for age and for improvised repairs.
10. Sea trial. Run at cruise long enough for temperatures to stabilise, then to full throttle. Confirm the engine reaches its rated maximum rpm.
11. Watch exhaust colour under load, temperature stability, vibration through the range, and gear engagement ahead and astern.
12. Commission an oil analysis and a compression or leak-down test before making an offer.

The tests worth paying for

Oil analysis — a laboratory test for wear metals, coolant, fuel dilution and water. Inexpensive relative to what it reveals, and frequently decisive.

Compression or leak-down — the most direct measure of internal condition without opening the engine. Leak-down additionally localises the loss to rings, valves or head gasket.

Injector testing on any engine with starting or smoke symptoms.

A marine engine surveyor, separate from the hull surveyor, on a high-value engine.

Reading the result

Findings are either deal-breakers or negotiation. A deferred heat exchanger service is a number you can price. A block with no records and unknown history is a risk that should be priced or walked away from.

See the full pre-purchase checklist and how long a marine diesel should last.

Frequently asked questions

Does a boat survey include the engine? Usually only superficially. Engine detail is typically a separate engagement.

Why does the engine have to be started cold? A cold start reveals starting difficulty, smoke and noise that a warmed engine hides. It is the most informative moment of the inspection.

Are low hours a good sign? Not by themselves. Low hours over many years often means long idle periods, which carry their own deterioration.

What is the biggest red flag? An engine warmed up before you arrive, combined with no service records.

Is a compression test necessary? On any significant purchase, yes. It is the most direct evidence of internal condition available without opening the engine.