Engine Checks & Preventing Fuel Contamination

Checking your engine before you set off could avoid breaking down when you are underway. Increased care is needed in the storage of diesel (gas oil) where this contains biodiesel.

Engine Checks

Checking your engine before you set off could avoid breaking down when you are underway.

Before starting

Look for signs of leaks which might indicate something more serious and inspect belts and hoses for damage. Make sure there is enough fuel for your journey and an adequate reserve in case your plans change. Check oil levels and keep top up. Some engines have fresh water cooling systems so make sure the coolant level is correct. Check stern glands for leaks and grease where necessary. Examine filters where it is possible to do so, particularly water separators in fuel lines.

Finally check the propeller is clear of anything that might foul it before starting.  

After starting

Immediately check that there is a good flow of water from the exhaust pipe or cooling water tell-tale in the case of an outboard.

Allow the engine to warm up so that it will tick-over smoothly before setting off.  


In recent years there has been considerable concern about biofuel in diesel fuel supplies. Increased care is needed in the storage of diesel where it contains biodiesel.  Further advice on fuel supplies.

Preventing Fuel Contamination

Increased care is needed in the storage of diesel (gas oil) where this contains biodiesel.

Due to their hygroscopic nature, biodiesel blends can contain more water than ‘normal’ diesel which can result in accelerated corrosion, sediment formation, and filter blocking. All of this can be controlled by good housekeeping and fuel management.


All diesel is contaminated with water to some extent either because it is suspended in the fuel itself or it gets into fuel tanks through faulty seals and vent pipes and from condensation caused by changes in ambient temperature. The latter is a particular problem in common rail diesel injector systems. Because biodiesel is hygroscopic, it exacerbates the problem and biodiesel blends are more susceptible to biological attack by micro-organisms. Aerobic micro-organisms that consume hydrocarbons, such as fungi, bacteria, and yeast, usually grow at the interface between fuel and water in fuel tanks. Anaerobic species can actively grow on tank sides.

Bacterial growth

Bacterial growth can result in the blockage of fuel pipes and filters and increase the problems of corrosion. Prolonged use of contaminated fuel may result in damage to engines. Bacterial growth can be prevented by eliminating water from fuel tanks and conducting regular checks to ensure that tanks remain free of water. Where a bacterial growth outbreak has occurred, this can be addressed either by emptying and cleaning the tanks, or by tackling the outbreak with biocide additives and filtering.


Biodiesel is a better solvent than ‘normal’ diesel. As a result it may pick up deposits already in fuel systems and in fuel tanks. To prevent those deposits from blocking filters, a one-time replacement of storage tank and off-road equipment fuel filters, outside the regular service interval, after 2 to 3 tank throughputs of biodiesel is recommended.

In addition, fuel seals in sight gauges on older fuel storage tanks may be incompatible with sulphur free diesel, irrespective of whether it contains biodiesel, and may require replacing. Users should examine seals and if there are signs of leakage, they will need a one-off replacement of these seals.

Oxidation stability

The oxidation stability of biodiesel is poorer than that of ‘normal’ diesel. Over time oxidation can precipitate solids with the potential to block filters in fuel distribution systems. To minimise the likelihood of this occurring, it is recommended that users take particular care to ensure a fuel turnover period of once every 6 months and, in any event, no longer than once every 12 months. Bio-diesel blends have a higher Cold Filter Plug Point (CFPP) than ‘normal’ diesel which means it may not flow as well (a phenomenon known as ‘waxing’) in cold weather or stop altogether. However, the fuels made available to the latest standards (BS EN 2869:2010) include additives to prevent waxing and maintain oxidisation stability.

Current advice based on good practice recommends that:

  • fuel in any tank is turned over regularly, at least every 6 months and certainly no more than 12 months;
  • when in use, tanks are kept as full as possible, to reduce condensation, however this must be balance against the amount you use and how long a tankful is likely to last you
  • water must be drained off regularly (although it is rarely possible to remove it all) in order to discourage MBC (micro biological contamination). Consideration should be given to modifying the drain facilities to make them more effective
  • seals and components in the fuel system are inspected and, where necessary, replaced
  • strainers and filters are checked and cleaned more regularly


It is understood that this is easier said than done. Smaller marinas and boatyards may only have one supply tank and may not sell enough fuel to turn it over regularly particularly in the winter months. Many recreational craft are laid up over the winter with full tanks for 6 months or more in some cases.

A balance must therefore be struck between the amount of fuel bought and the amount of fuel you use. Where possible you should try to buy diesel that does not have biodiesel in it.  But remember that the problems described here also affect ‘normal’ diesel as well, albeit to a lesser extent.

If you are concerned about biodiesel and whether there is something nasty in your tank, test kits are now available, which can identify whether contamination is present and its severity. These have been demonstrated to give quick and accurate results on-site.

Related pages

Diesel Engine Course