Overheating in a marine diesel engine can be caused by a wide number of factors, but as with the diagnosis of all faults it’s important to check obvious and easily rectified causes before investigating more deeply. A surprisingly common and easily overlooked one is failing to open the cooling water seacock after working on the engine.

It’s important to understand the elements of the cooling system on your boat – two different types are commonly fitted to marine diesel engines. Directly cooled units have raw water (the water the boat floats in) pumped around the engine. Indirectly cooled engines are a more complex, with a closed cooling system somewhat like that of a car. However, in place of the radiator there's a heat exchanger in which the coolant in the sealed part of the system is cooled by raw water.


Marine diesel engine belts should be tensioned so that 10kg of thumb pressure results in around 10mm of movement on the longest run between pulleys.

Drive belt problem with your diesel engine?

Water pumps are normally powered by a drive belt on the front of the engine, which can be checked in a couple of seconds (with the motor turned off to prevent risk of injury) in the event of a problem. Belt wear accelerates rapidly if they are too loose and able to slip. Belts should be tensioned so that 10kg of thumb pressure results in around 10mm of movement on the longest run between pulleys.

Marine diesel engine cooling water flter

Check your diesel engine cooling water filter for debris.

Blocked intake?

If the drivebelts are okay, next check that the seawater filter is not blocked by debris. This may be an integral part of the skin fitting, or a separate unit attached to the pipework between the skin fitting and the engine. After clearing a blocked filter start the engine and look for a flow of cooling water from the exhaust – this may take 20 seconds to flow. If you’re lucky the water pump impeller won’t be damaged, but if the diesel engine was run for more than a minute or two without cooling water this may have suffered damage and should be inspected as soon as possible.

Even if the filter was not blocked it’s possible for a plastic bag to be sucked over the intake if the boat is stationary or moving slowly. This can be a challenge for fault finding as the bag may float off once the diesel engine is shut down, leaving no evidence of a problem other than a burnt-out impeller.

Water pump impeller

The water pump impeller on a marine diesel engine is easily damaged.

 Impeller damage

The rubber impeller in the sea water pump requires water for lubrication and can be damaged if it runs dry for as little as one minute at high revs. Equally, the vanes of an ageing impeller will adopt a permanent deflection that reduces the pump’s efficiency, before eventually breaking apart. Remove the faceplate of the pump to inspect it and replace the impeller if damaged, removing any broken elements of the old one before fitting the replacement.

If all else appears to be okay with an indirect system check there is sufficient coolant in the heat exchanger, waiting for the system to cool before removing the filler cap. Restart the engine and if possible run it under load for a few minutes. If this has not solved the problem, a flow of water from the exhaust indicates the raw water pump is functioning, however indirect cooled engines have a second pump that circulates coolant around the engine and heat exchanger. The same belt as the alternator often drives this, so this belt should be double-checked and the impeller inspected for problems.

Marine diesel heat exchanger

Check the heat exchanger.

 Problems with your marine diesel engine at high revs

Overheating may occur when a marine diesel engine is run at high revs for an extended period, which puts systems under more strain than they normally encounter, exposing latent weaknesses. This may be the case if no other problems can be identified – if so after allowing the motor to cool it may be possible to continue at modest speed without further problems. However, the cooling system should be checked thoroughly and any underlying problems rectified before the boat next puts to sea.

Over time marine diesel engines tend to be subject to narrowing of the cooling waterways due to a build up of limescale, which will reduce the efficiency of the cooling system. This happens within the engine of directly-cooled engines, or in the heat exchanger of indirectly cooled versions, which is more easily solved, but is still not a job that can be readily undertaken at sea.

Thermostats rarely cause trouble, but if it’s stuck closed the engine will overheat within minutes of starting. Thermostats are easy to remove and engines can be run short term without one, although engine wear will increase, and fuel economy will suffer, so a replacement should be fitted as soon as possible.

After the event

If a marine diesel engine has overheated it’s always worth checking the condition of the impeller and keeping an eye open for evidence of head gasket problems, particularly water in the oil (usually seen as a milky emulsion under the oil filler cap). Overheating can also cause inner walls of the rubber sections of the exhaust to delaminate, which can restrict the passage of cooling water and exhaust gasses.

Engine control panels

If the diesel engine is not charging batteries the warning lamp will light, but with many engines the alarm buzzer will not sound

Charging problems

If the marine diesel engine is not charging batteries the warning lamp will light, but with many engines the alarm buzzer will not sound – unlike overheating or low oil pressure, if there are no other problems the engine will continue running without further damage, so there’s no reason to sound an urgent alarm.

First check the alternator drive belt to confirm it’s not excessively worn, loose or broken. Drive belts are at optimum tension when 10kg of thumb pressure on the longest run between pulleys will deflect the belt by 10mm.

If the belt is okay the extent of the problem can be confirmed by checking the voltage of the batteries with the engine running. Ideally this will be above 14V, although lower figures may be seen if the batteries are large compared to the alternator and are very discharged, or if they a charged through split charging diodes.

Open circuit battery voltage

If the voltage is less than 13V, the battery is probably not being charged.

However, if the voltage at the batteries is less than 13V, the battery is probably not being charged. If you're on passage it's worth conserving as much electrical power as possible, while monitoring remaining charge by checking the voltage at regular intervals. With no load being drawn a well-charged battery will show 12.6V or more. A figure of 12.2V represents a battery that's 50 per cent discharged, and at 11.8V it is more than 90 per cent discharged.

These are rough and ready tests – ideally a battery should be left for several hours with no load drawn from before using a voltmeter to check its state of charge. That’s obviously impractical when problems are encountered at sea, but in the absence of more sophisticated battery management electronics, periodically measuring the voltage will give a good indication of the rate of discharge of the batteries, providing the load on them is reasonably constant.

For more on other marine diesel engine faults see  Diesel engine repairs: fuel, air, starting, wiring and Diesel engine maintenance tips

Rupert Holmes’ iPhone app – Marine Diesel Expert – is designed guide users through the marine diesel engine’s key systems in logical easy-to-follow steps, helping boat owners diagnose problems and rectify the most common marine diesel engine faults.