Bleeding the engine

If a diesel engine pegs out under load and it hasn't overheated there is probably air in the fuel system. The only other likely explanation is that it has suffered some major mechanical failure. There'll be no doubt if that is the cause, because there will be dramatic manifestations in the form of noises, fumes and smells. Fortunately, air in the fuel is far more common. It is also readily dealt with.

Your fuel system usually becomes air-locked because either you have run out of fuel, or you have clogged-up filters as a result of dirty diesel.

It's easy to ascertain whether or not air is your problem. At the end of the fuel chain are the injectors, which are easily recognised. They are on the side of the cylinder head and look something like all-metal spark plugs. Follow the pipes back from them to the fuel injection pump which is generally bolted to the side of the engine. Inspect this and you'll find one, or possibly two, bolt heads apparently doing nothing much at all. These are the bleed screws. Ease them off a turn or two. If the system is choked with air, bubbles will froth out. The airlock which they indicate must now be systematically expunged. This is achieved surprisingly easily by following the fuel in its pipe all the way from the tank to the cylinders of the engine.

The first filter is usually positioned between the tank and the body of the engine. This filter does most of the work and is usually the one that is clogged up. Turn off

Bleed points

L U Lft

^- Injector Pump

Bleed points

Lift pump

Bleed points

1\

■ I

Tank

II

Fig 9.1 Bleeding a diesel engine.

the fuel tap at the bottom of the tank and take the filter apart, cleaning it out and replacing the disposable element - if there is one - from your well-stocked engineer's stores. This won't take you long if the filter is accessible. Have an old towel by you to mop up the excess diesel; as well as a small plastic container to collect the main spillage. If the filter is inaccessible, I suggest that you either move it or buy a new yacht from a more caring manufacturer, because sooner or later you're going to have to do the job when it really matters.

The top of the filter will have a bleed screw like the one on the pump. Reassemble the filter, turn on the fuel, then ease this away until bubbles start foaming out. If the tank is lower than the filter, you may have to work the small lever on the side of the fuel lift pump (see below) to induce a flow. When clear diesel runs out, tighten down the screw. The system is now bled this far. If by any chance there is no bleed screw, crack the feed pipe union at the side away from the tank.

The fuel pipe's next stop is at the lift pump, unless there is more than one prc-filter. The engine usually operates this by mechanical means, but when the main power unit is not functioning, you must turn the engine with the starter motor or by hand (a decompressor helps). If this is unattractive on account of anxiety about your battery or your heart, you can activate it perfectly well with the small lever on one side of it, which enables you to pump fuel manually.

There is usually a final filter between the lift pump and the injector pump. This filter is not often dirty if the first one is working well, but if you have any doubts, strip and clean it anyway. Whether or not you do so, you will certainly have to bleed it in the usual way. From here, the fuel pipe runs to the all-important injector pump. Bleed this as well. Now you are ready to start her up. The injectors will bleed themselves, but it may take some seconds of engine-cranking before they are doing their work successfully.

Find the bleed points on your engine. This one is typical. The main one is at the top of the fuel filter.

If you arc imprudent enough to run out of diesel, you must go through the same process. You may think you'll never be so stupid as to let this happen, but sooner or later, everyone is caught out. In my case it resulted from a blocked-up sight gauge. I knew I had enough diesel to get back to my mooring until the engine stopped. I was left drifting around the English Channel in a glassy calm until a passing Frenchman exchanged 5 gallons of the finest for a quantity of good English ale.

Try to make sure that when your turn comes, bleeding the fuel through after you have filled the tank from your carefully carried emergency cans is mere routine. Don't wait for a moment of life and death to find out whether or not you can do it. Try it first in your berth on a foul day when you don't feel like going for a sail. You only need the simplest tool kit. Ultimately, you'll be glad you did, because there are no mechanics offshore waiting for your call. And if the engine ever stops on a dark and stormy night, you'll be confident that you know exactly what to do, regardless of what boat you arc on.

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