Keep it clean
Diesel systems need to be checked and have new filters fitted regularly, especially with fuel bug issues. Here’s how
When it comes to servicing the fuel system most people think of the diesel mechanical parts, but just as it takes two to tango it takes two for an engine to work properly – fuel and air.
Blocked air filters or other forms of air restriction strangle engines and produce black smoke, so if your exhaust is smoking and you find your filter is badly clogged, simply take it out and run without one until you can either clean it or get a replacement.
‘Pancake’ or ‘frying pan’ filters are the simplest to deal with, they just need the old ‘paper’ element removing and replacing. There’s usually either a central wing nut or clips around the cover holding the parts together.
Mesh filters require a little more work, their wire or plastic foam needs to be removed and washed out in white spirit or paraffin, dried and then coated with a thin film of engine oil to trap the dust. Don’t be tempted to renew a foam filter by cutting up odd pieces of foam you happen to have lying around, they must be made from open cell foam that allows air to pass through.
Not all engines have air cleaners. Some Vetus engines look as if they have one, but it’s actually only a silencer to quieten the noise of the air intake. Some older Perkins and Listers simply had a sort of metal ‘dome’ to stop you dropping things into the air intake and some Isuzus might not even have that.
If there’s a hose running from the top or side of the engine into the air cleaner, or a point very close to it, take the hose off and either clean it out or replace it. Also make sure any metal connecting pipes are clear.
Moving on to the diesel aspect, first, switch off the fuel tap (it’s easy to forget…) and place a bilge blanket under the water separator (if you have one) and filters.
A water separator is usually the first ‘filter’ in the fuel line from the tank, so remove the bolt holding the bowl on and try to catch any diesel that runs out in a tub or bucket.
Pour the remaining contents into the tub and inspect it for signs of water (globules moving across the bottom), cloudiness any slimy grease/jelly like substance. The latter two indicate that fuel bug may be present, even if you are not suffering any problems. Gritty particles are rust or dirt and show that the unit is doing its job.
Wash the components in clean paraffin or white spirit and dry them with a lint-free cloth, then reassemble, renewing any rubber seals as necessary.
If the fuel pump has a filter or strainer move the tub and blanket beneath the pump and clean them, otherwise place it below the engine fuel filter(s).
On a modern engine these are likely to be ‘spin-on’ types, but older engines might have a paper element inside a bowl or a metal/paper element sandwiched between the filter head and a shallow bowl. Both are dealt with in the same way except that the ‘sandwich’ type has rubber seals above and below the element, while the bowl type only has one seal in the filter head. The bowls of the latter two are retained by a centre bolt at the top or, occasionally, at the bottom of the assembly.
When you’ve removed the filter, empty fuel from it into the tub and inspect the filter and tub for signs of diesel bug or water. Fill the new filter with clean fuel
and screw it on until it touches the head and then another half to three-quarters of a turn by hand.
If your engine has a mechanical fuel pump remove any screw(s) holding the cover onto it and remove the cap, ensuring you don’t damage a rubber sealing ring. Remove the strainer and clean. If there is a sediment trap below the strainer clean it with lint-free cloth and (probably) a small screwdriver. Check for signs of diesel bug or water.
Some Mitsubishi-based engines (and possibly others) use an electric fuel pump with a small filter in one end. It’s vital that this filter is changed regularly – especially if there is no water trap. This is done by unscrewing the ‘bayonet’ end cap and removing the small element. There is also a magnet under the cover that requires cleaning. Check the filter and housing for signs of water or diesel bug.
Once it’s reassembled, if your system is said to be self-bleeding and your battery is in good condition, turn the fuel tap on and operate the starter in 30 second bursts. Eventually the engine should fire and run – providing the battery holds out.
If not and you are lucky, you will only have to bleed the low-pressure side of the system. However occasionally you may then have to go on to bleed the highpressure system and you certainly will if you run out of fuel for any reason.
First check the manual and read what is it says about bleeding the fuel system. If the boat has an electric fuel pump and a small pipe leading from the filter or injector pump back to the tank, you are in luck. Just turn on the fuel tap and ignition and the electric pump will drive the air back into the tank.
For manual bleeding, it might be possible to bleed any water traps by gravity, otherwise you will have to pump the air out of the system via the filter bleed point.
To bleed the water trap by gravity, loosen the bleed point on the trap, if it has one, or the outlet pipe union. Spit around the loosened point and watch for bubbles of air coming out. Once no air is coming out, tighten pipe and move on to bleeding the filter(s). When fuel leaks from the bleed point or union, tighten it and clean any spillage.
To bleed the filters, loosen the bleed point on the filter head and operate the priming pump. First air should come out of the bleed point, then bubbles, then fuel interspersed with air and finally just fuel. Then tighten the bleed point.
If you cannot get pure fuel to flow you probably have a faulty seal or soft washer on the water trap or lift pump cover.
Modern engines usually have a self-bleeding injector pump, but they can still be bled from the point where the pipe that returns to the tank is fitted to the injector pump. Bleeding it if necessary is much the same as the fuel filter.
If you need to bleed the high pressure system to get the engine running, loosen the injector pipe nuts at the injector end, half a turn is adequate, spin the engine on the starter and watch the pipe unions. When they ustart to spit or drip fuel tighten them. The engine should now start.
Bleeding the fuel has probably the largest potential for problems, so if you are at all unsure get someone to show you how to do it (even if the manual says it’s self-bleeding).
Change paper air cleaner element
Gauze-style air cleaner
Check water traps
Priming arm on fuel lift pump
Change the fuel filter