Engine Oil Questions

At least once a year or if the engine is in regular use, particularly if it's heavy use, more frequently.

New engine oil is clear, used oil is black. Ever thought why? The engine has many moving components, many work with small tolerances between the parts. As the oil is moved around the engine, it leaves a film of lubricant on the parts to enable smooth movement and also helps dissipate the heat generated. It also collects any particles of wear and carbon from the engine which should get collected by the oil filter so that they are not transferred back to the moving parts. These particles are what tend to turn the oil black. Over time, the oil will also pick up moisture from the damp environment which is not good for the engine either. Changing the oil regularly gives you one of the best indications as to the condition of the engine without stripping it.

15w/40 Diesel Engine Oil

Yes. Change the oil filter each time you change the oil. If you don't any particles that are in the filter may be lifted and distributed around the engine with the new oil.

If you have an engine that's in great condition, please follow the owner's manual or look out for a plate on the engine notifying you of what oil should be used. If you have an older engine, the oil choice may be less precise due to wear and tear. You might need to choose a thicker oil to prevent it entering the combustion chambers. We tend to use a quality 15w/40 diesel oil in our BMC powered fleet vessels.

These engines tend to require frequent oil changes. If it's little used, probably every six months. If it's used regularly, as much as every two months. In hire, we would recommend every six weeks on a 4017/4018!

It's come from somewhere, so find out where. Many older engines tend to leak a little oil, BMC 1.5 and Perkins 4108 engines are famous for it. Are you regularly checking the dipstick to see if your oil level is reducing regularly? If you have an oil cooler, check the pipes to and from it for leaks - failures here can be catastrophic. Remember also, oil floats on water, so you night have a thin film of oil on a lake of water. If so, you need to find out where the water is coming from. Clean it up (safely), and monitor it.

Too much oil in an engine can be as problematic as too little. Too much oil increases oil pressure which can cause the engine to burn oil as it's forced past the oil seals in the combustion circuit and also the oil seals on the main crank, all of which may be damaged by this process. You should remove any extraneous oil until you reach the right level on the dip stick. Also, question where the oil has come from. Is it oil? Water might have found its way in and, on older engines particularly BMC and Perkins (see our BMC 1.5 blog), the modern diesel will wear away the seals in injector pumps which will then leak into the lubricant oil causing the level to continually rise. The worst possible scenario for this is the engine beginning to run on the sump fluids and destroy itself as well as being potentially dangerous to anyone onboard or close by.

DO NOT RUN THE ENGINE! White, creamy engine oil is a sure indication that the oil has mixed with water. Serious engine wear may occur if you run it which could result in a total seizure and huge repair/replacement bills. Possible entry points for the water are: broken cylinder head gasket or cracked cylinder head, possibly a cracked engine block, leaking raw water pump, submerged engine (or partially submerged if crank oil seals are perished) and other possibilities.

Other Questions

Poor starting can be caused by many problems. If the engine will run well enough once you have started it, the problem is most likely related to the Pre-heating circuit or reduced compression in the cylinder(s). Incorrect injection timing can also be a further issue as well as injectors and lift pumps. You can check your glow plugs across a battery (never a battery charger) but be sure that you are testing 12v plugs; many older engines used lower voltage plugs which will instantly burn out on 12v.

Kingfisher Boat Services can run a compression test on your engine and help you diagnose starting problems

This depends on the colour of the smoke.

  • Blue smoke is burning engine oil. This indicates a worn engine that requires an overhaul.
  • Black smoke is poor combustion. This may be down to blocked air filters, faulty injectors, worn turbo and many other possibilities.
  • White smoke is incomplete combustion of the fuel and/or water vapour. This may be incorrect injection timing, faulty injection pump and/or injector(s) or water entering the combustion process indicating a failed head gasket or worse.

Any smoke from a diesel engine (once it's warm) should be taken to indicate a fault which should be investigated.

BMC 2.5 Diesel Marine Engine
So long as there was no associated unpleasant noises, it's likely that you have a fuel problem. Firstly, if the engine turns normally when you try to start it, internal damage is probably not the cause. Make sure you've not run out of fuel, check the fuel filters for dirt, water and sludge, bleed the fuel system and try to restart.

We would recommend at least once a year assuming regular use. However, if your fuel is particularly dirty or you find an unacceptable amount of water in the filter bowls, you should check more regularly. Bad fuel is a common breakdown issue and it's easily prevented with regular maintenance.

This is probably a fuel system fault. If it's a regular minor "cough" check that it is followed by a puff of smoke from the exhaust; if so, it's probably a sticky injector. If it's more of a sudden rise in revs and then settles or perhaps even dies, it is most likely an air leak somewhere in the engine's fuel system.

Overheating will eventually kill even the most resilient of engines such as a BMC and could destroy a modern alloy engine very quickly. In fact, if you have a modern engine, regular overheating may already indicate serious damage. Check the obvious - water pump belts, impellers, weed filters and hoses. Remove the oil filler cap and check for white, emulsified oil (indicating a blown head gasket or cracked head/block). Remove the thermostat and test / replace. A cylinder head problem won't always manifest itself with emulsified (white) oil. Other tell-tale signs are pressurisation of the cooling water circuit.

Let Us Help

Kingfisher Boat Services has a wealth of experience in dealing with all aspects of inland marine craft.

Call us in to assist with your boat's problems for a robust solution. We can work on an advisory basis or carry out the whole job to completion; the choice is yours.

For many, DIY tasks on your boat are all part of boat ownership and carrying out these jobs is all part of the fun, but there often comes a time when you simply can't spend the time or the job is just outside of your skill level or comfort zone. Perhaps you need specialist tools (sorry, we don't hire tools), or specialist knowledge where to proceed further without it might cause greater problems.

Petrol inboard engines are particularly dangerous to work on because of the very real risk of explosion from igniting the fuel vapours.


Please use the information on our Frequently Asked Questions pages as you see fit but at your own risk. We have a wealth of experience in these matters and are happy to offer this information free of charge, but by using any of details given on this page or other pages within this web site, you agree that you are doing so at your own risk. Kingfisher Boat Services cannot be responsible for any losses caused as a result of following any information given here.