Boat engine fuel system

If engines are installed and serviced correctly then most marine engines are very reliable, but they are not infallible and problems are more likely to occur if an engine is not taken care of and inspected from time to time, in addition to carrying out the all important engine servicing. One of the most important parts of the engine to check and service is the fuel system.

Fuel system checks

Regular checks of the fuel system are necessary throughout the year, especially the primary filter of a diesel engine fuel system. With the boat in the boatyard, it is a good opportunity to replace the fuel filters and then bleed the system to remove any air that might have entered the system as a result of replacing the filters. You will need to buy replacement filters as recommended by your engine manufacturer in advance.

Primary filter – this catches any water and dirt that might be in the fuel. Typically, this has a filter beneath which is a sealed glass bowl where dirt and water get trapped. To replace the filter do as follows:

1. Make sure the tank fuel stopcock is closed.
2. Diesel is harmful to the skin, so wear protective gloves in case of fuel spillage (which is more than likely to occur). Also have some rags or paper towels at hand.
3. Use a suitable container such as a glass jar or plastic container to catch all the fuel which will drain from the filter and bowl.
4. Undo the drain screw at the base of the glass bowl to drain off the fuel into the jar.
5. Now undo the central retaining bolt that holds the glass bowl and filter in place, carefully separating these from the retaining bracket above. There are normally rubber O-ring type seals between the bracket, filter and bowl. New seals are normally supplied with filters – it is best to check when ordering.
6. Clean all the parts, paying particular attention to all the contact surfaces
7. Reassemble with the new parts, including the new seals, taking care not to over tighten the bolt securing the bowl, filter and bracket together – only moderate pressure is required. Smear some diesel on the seals to help them bed in properly.
8. Open the fuel stopcock and check for leaks.

Secondary filter – also known as a fine filter, the secondary filter is needed to separate the extra fine particles in diesel fuel that could damage the injectors. It does not need replacing as often as a primary filter, normally manufacturers recommend doing this after 200 hours’ use. There are two main types of secondary filters: spin-on style where the filter is contained in a metal canister and cartridge type filters where only the filter medium is replaced.

The replacement procedure for a spin-on filter is as follows:

1. Make sure the tank fuel stopcock is closed.
2. Unscrew the filter a half turn using a filter wrench, pipe wrench or strap.
3. Now hold a plastic bag around the filter and continue to undo it by hand, being aware that it will be full of fuel. Then dispose of it in the boatyard’s recycling containers.
4. Replace the rubber seal, smearing a little diesel fuel on the new seal to help it bed in.
5. Carefully pour some diesel fuel into the new filter. This will help make bleeding the fuel system quicker. Note, to prevent spillage do not attempt to do this with a primary filter as in my experience assembling the new filter, seals and glass bowl is tricky enough without the new filter having fuel in it.
6. Screw on the new filter and hand tighten it only.
7. Bleed the fuel system and check for leaks.

 Bleeding the fuel system

When you replace the fuel filters, air gets trapped in the fuel system. This also happens if you run out of fuel. When you try to start the engine the trapped air prevents the fuel from flowing properly, hence the engine will run badly and may not start at all. This happens because if air bubbles get into the injector pipes the injector valve does not operate. Some engines do not require this as they are self-bleeding, but many do. These are the steps to take to bleed a fuel system:

1. Turn the fuel tank stopcock on.
2. You need a container and rags to catch drips from the fuel system as you bleed it and a small screw driver and/or spanner as required. If in doubt you may need to consult the manual. Wearing protective gloves is also a good idea.
3. Identify where the hand priming lever is on the fuel lift pump and where the bleed screws are on the filter bodies and injection pump. These can be a little tricky to find for the first time in a cramped engine bay.
4. If the primary filter is at a lower level than the fuel in the tank undo the bleed screw. Note you do not need to unscrew it completely.
5. Fuel with bubbles will begin to appear, so hold a container beneath the filter to catch the drips. When the fuel runs clear, close the bleed screw.
6. Wipe the primary filter dry.
7. Unscrew the bleed screw on top of the secondary filter.
8. Operate the hand priming lever on the fuel lift pump. If the lever won’t move, this will most likely be because the engine will have stopped where the pump is at the end of its travel. Try turning the engine over a little either with the starter or by hand, then try the pump again.
9. Keep working the priming lever until clear fuel comes out with no bubbles and then tighten the bleed screw.
10. Wipe the secondary filter dry.
11. At this point the system should be bled and the engine should start but if not the injection pump and high pressure line may need bleeding.
12. Bleed the injection pump next, having located the upper bleed screw.
13. The last parts to bleed but only if necessary are the injector pipes. By this point an extra pair of hands will be of help to operate the starter.
14. To do this, slacken the pipe connectors that join the pipes to the injectors one by one, cranking the engine in short bursts and checking for fuel escaping from the unions. Take care as fuel escaping from the injection pipes will be under very high pressure.
15. Finally re-tighten all the fittings through the system, shut off the fuel tank stopcock, wipe everything dry and dispose of any spilt fuel.

A final note on this. Some boat owners prefer to wait and bleed the fuel system when the boat is back in the water. This enables the engine to be run for a while and checked over with the cooling system operational.