Troubleshooting boat engines, sterndrives and outboards

Choose a symptom. Start with the simple checks and stop when the job needs a workshop.

Starting problems

Boat engine won't start after the winter

The engine that ran last autumn won't start this spring? Check the battery, fuel valve, old fuel, filters, spark plugs and seacock, in that order.

Boat engine won't start when warm

Does the engine start cold but not warm after a stop? Check the starting procedure, choke, fuel quality, spark plugs, battery and starter motor for a warm engine.

Air in the fuel system – the diesel will not start

Diesel won't start or stops after a filter change or empty tank? Check the fuel and prefilter, bleed with the hand pump and look for leaks on the suction side.

Engine won't start

Tell a starter motor that won't crank apart from an engine that cranks but won't fire. Check the controls, battery and fuel before calling in a workshop.

Starter motor clicks or does not crank

Does it click or stay completely silent when you turn the key? Check the main switch, neutral position, battery, cables and fuses before the starter motor.

Runs rough, stalls or loses power

Boat engine dies at idle or when shifted into gear

Does the engine die at idle or when you shift into gear? Check the tank vent, filter and water in the fuel, idle speed, propeller and shift cable, in that order.

Boat engine hesitates or dies on acceleration

Find out why the engine stutters, runs rough or dies when you open the throttle: tank and vent, water in the fuel, filters, air intake and spark plugs.

The boat engine knocks or makes strange noises

Trace knocking, ticking and vibration in the engine: oil level and oil pressure, temperature, belts, engine mounts, octane rating, valve clearance and injectors.

Boat engine stops after a while

Engine runs for a while and then stops? Check the tank and valves, tank venting, pre-filter and pickup, air leaks in the hoses and the overheat protection.

Boat engine does not reach full revs at full throttle

Find out why the engine does not reach full-throttle rpm: tachometer against the manual, throttle cable, load, fouling, propeller pitch, trim, filters and exhaust.

Smoke and exhaust

The boat engine smokes blue – burning oil

Blue smoke means the engine is burning oil. Check the oil level, crankcase ventilation and two-stroke mixture before the workshop looks at the turbo and piston rings.

White smoke from the boat engine exhaust

Distinguish harmless cold-start vapour from white smoke that persists. Check cooling water flow, thermostat, water in the fuel and signs of water in the cylinders.

Diesel engine emits black smoke

Troubleshoot black smoke from the marine diesel. Start with load, fouling and propeller, move on to the air filter and air intake, and leave injectors and turbo to a workshop.

Engine smokes or loses power

Troubleshoot smoke and power loss without swapping the turbo on a hunch. Start with load, propeller, air, fuel and oil level before a workshop diagnosis.

Cooling and temperature

Engine not cooling – weak or no cooling water

Troubleshoot weak cooling water flow, from the intake and raw water strainer to the impeller and thermostat. Checks from Volvo Penta and Mercury MerCruiser.

Temperature alarm – engine overheating

How to handle temperature alarms and tell low coolant apart from raw water flow problems. Start safely and avoid continuing to run the engine with an alarm.

Oil, water and fuel

Petrol smell in the boat – fuel leaking from the engine

Does the boat smell of gasoline? Ventilate first, do not start. Then go through the hoses, primer bulb, fittings, tank vent and filler to find the leak.

Water in the fuel or tank of the boat engine

Find water in the fuel: drain the water separator, take a sample from the tank, avoid condensation and watch out for diesel bug and phase separation in E10 petrol.

Water in the engine oil – the oil is grey or milky

Grey or milky engine oil means water in the engine. Tell condensation from a real leak, check whether water has reached the cylinders and when the workshop should take over.

Electrics, alarms and fault codes

Battery not charging – charge warning light is on

Charging light on or voltmeter reading too low? Check the drive belt, cables and battery, measure the charging voltage and inspect the charge splitter before the alternator.

The alarm is sounding – what do the warning lights mean?

What do the oil pressure, temperature and charging lights mean on a Volvo Penta, and why is the outboard beeping? How to test the alarm and what to do for each.

Oil pressure alarm on the boat engine – low oil pressure

Oil pressure light on? Stop the engine, check oil level, oil grade and filter, and let the workshop determine whether it's the sender or the pressure.

Volvo Penta fault codes (EVC) – what do they mean?

Read the fault code on the D4/D6 with EVC, look it up in Volvo Penta's fault code register and decide whether you can continue at low speed or must stop the engine.

Drive, gearbox and trim

Drive knocks or vibrates

Troubleshoot unusual noises without confusing engine and drive faults. Check the propeller and visible mounts before workshop work on joints, bearings and the drive.

Milky drive oil or oil leak

Milky drive oil can mean water ingress. Check the level and look for visible leaks safely, and have a workshop find the cause before running the boat again.

Trim not working – the drive won't go up or down

Distinguish a trim motor that doesn't run from a motor that runs without the drive moving. Check fuses, battery, trim fluid, manual valve and trim gauge.

Gear will not engage or jumps out

Check the control's neutral button, shifting technique, cables and linkage, the oil level in the reverse gear or saildrive and electronic shift faults before going to the workshop.