To visit this company's website, click the banner below.

A Guide to Basic Diesel Engine Maintenance

5th February 2020

Diesel engines are relatively straightforward to maintain and if you can cover the basics on a regular basis then you can be sure to prevent costly mechanical failures that could leave you, quite literally, at sea.

The fact is that the marine diesel engine is placed under unique environmental stress but is relatively easy to maintain to keep at optimum levels of performance

This simple guide is designed to give you the basics to performing routine maintenance and regular checks on your marine engine to ensure long service and the best efficiency. You don’t need a comprehensive knowledge of mechanics to do this but if you are unsure then we would always recommend the advice of experts.

Fuel system

The routine maintenance of the fuel system involves changing fuel filters and bleeding the system.


Filters are an essential component of the fuel system in order to prevent both foreign particles and water from ingression to the engine. The majority of marine engines have a dual filter system; a primary filter on the fuel lines between the engine and the tank with a secondary situated between the injection pump and the fuel lift pump.

The primary filter on most boats will consist of an element with a small bowl beneath to collect sediment and water. This bowl should be checked and drained (when required) on a regular basis. This is a simple process of loosening the plug or cock at the bottom of the bowl and allowing the build-up to run out until clear fuel passes through. Ensure that you have a suitably sized container to collect the volume that needs to be drained.

The filter itself will typically need replacing every 200 run hours (check your manufacturers manual) and can be done without specialist servicing as long as you do the right preparation. Before you start, make sure that you have something beneath the filter to collect any spillages (a bowl is good but if space does not permit then you can suspend a sturdy plastic bag to collect any excess fuel).

Close the stopcock(s) to prevent fuel from escaping from the tank and unscrew the bolt which attaches the filter to the holding bracket. Before you replace the filter, check the condition of the sealing rings around the bowl and bracket. If they have started to perish then replace these at the same time (replacement filters are usually supplied with new seals so we would recommend doing so regardless of the state of the old ones). When changing the seals, smear a small amount of diesel around them helps them to bed in more securely when you tighten them

Fit the new filter by assembling it according to the instructions in your manual and reverse the above process. Make sure that you tighten the holding bolt securely.

Secondary filters can be similar to the primary filter and the process will be the same but many are of a spin-on variety. In which case, simply unscrew the filter with a filter wrench and replace. Before you begin with the secondary filter you will want to loosen the screw on the secondary filter bleed until the air has been released from the filter.

Bleeding the system

The process of changing the filters can allow air into the system and, in order to allow the engine to run properly you will need to purge the air under a small amount of pressure at the highest points in the fuel system. Whilst it’s true that some engines are vent air without intervention most will need a few steps to help this process along.

Ensure that the fuel lines are fully open and that the tanks have sufficient fuel before opening the bleed screw on your primary filter. When the bowl fills with fuel without bubbling then close it. Move on to the secondary filter and repeat the process. Bear in mind with the secondary filter that this can take a few minutes before being purged of air, particularly if your primary filter is filled in this way.

Some engines also require you to prime the injection pump after bleeding and you will need to unscrew the bleed screw to do so. It is worth remembering that some older models of pump have two screws and you should bleed these one at a time with the bottom one first.



You should regularly check oil levels, change at recommended intervals and replace the oil filters.

Engine Oil

The intervals recommended for oil changes vary depending on the engine manufacturer but it is recommended that this is carried out annually for good measures. It’s practical to do this at the end of a season to ensure that the correct additives are in the system to prevent idle damage such corrosion inhibitors as old engine oil can form sulphuric acid which can cause the engine to rust over the winter months.

The majority of engines will have a sump plug that you can access to drain the oil whereas others have a fitted pump such that you can pump out the oil through the dipstick hole.

When changing the oil you should first allow the engine to reach its normal operating temperature which allows the oil to reach its thinnest consistency plus collect any sludge from the sump so that this can be drained with the oil.

When changing the oil you should also change the oil filter and this should be done once the oil has been removed.

Always check your manual to ensure that you replace the oil with the correct quantity and grade of oil required to keep your engine running at optimum performance. It is always recommended that you add just a touch less than that which is suggested as it’s almost impossible to fully clear the system of the old oil. You can always top up later once it has settled; simply check the levels using the dipstick and fill as required.

Gearbox oil

Less crucial to change as regularly as engine oil because of the lack of contaminants it is a good idea to include the change of gearbox oil to renew the dosing of corrosion inhibitors which at the same time purges any particles which are in the system that could cause damage to the gears.

Before you start (and even if you do not intent to replace the oil), use the dipstick to draw out some oil to test the colouration. It should not appear black or a milky white; these suggest that you may have an overheating problem or water ingress respectively. These would need rectifying prior to changing the oil. The oil you replace should look clear and clean and you will most likely need to pump it out through the dipstick hole. As with the engine oil, check your manual for the type of replacement oil and quantity you will need.


Batteries are supposed to be almost maintenance free but corrosion around the terminals can occur and you should clean with hot water and bicarbonate of soda before rinsing. Top up with de-ionised water if needed and, when you are wintering your boat, then keep it trickle-charged.

Cooling system and drive belts

You should regularly check fluid level check and include an annual drain, flush and refill of the coolant at the beginning of winter. This is recommended to make sure that the anti-freeze levels are sufficient to provide adequate protection against the cold. Please remember that you should only change the coolant when the engine is cold as pressure can build up and vent boiling hot steam and water when the filler cap is opened.

With a freshwater system you should drain the coolant into a pan from the drain cock. Never allow the coolant to run directly to the bilge as antifreeze is toxic to marine life. Once drained, you can close the cocks and refill with the correct mix of coolant. The ratio of antifreeze to water will vary depending on the operating and idling temperatures in your area but the manufacturer will state this on their packaging. After you have filled the coolant you should run the engine for at least thirty minutes before checking the coolant levels and topping up where necessary.

Raw water systems should be flushed at the end of a season and the circuit plugged and treated with anti-freeze. While the engine is switched off Don’t forget to check all drive belts for wear and tension consider carrying some kind of link belting as a temporary replacement if needed

Air intake system

Whether air to your engine is filtered through an old oil screen, paper or synthetic filter these need to be cleaned and/or replaced regularly to prevent deposits from building up and preventing the free flow of air to your engine.

Other checks

• Routine visual inspections should look for signs of corrosion and leaks around the casings, mounts, valves and exhaust. • If you have sacrificial anodes mounted to protect against electrolytic corrosion then these should be checked regularly and replaced where necessary. • If cooling water can enter the exhaust because your engine is mounted low enough the anti-syphon valve can become crystalised with salt. This should be cleaned regularly with fresh water.

And finally, for all of your filters, inhibitors, oils and maintenance requirements ordered online and delivered quickly go to


Other engine spares and supplies on FAFB

Remember other advertisers on can supply you with what you need for hassle-free diesel engine maintenance.

If you found this article interesting please share it with others who may like it.