How to purge the fuel system of your boat's diesel engine

Oct 04, 2023

The fuel system of a diesel engine is sealed and must remain sealed. However, there are times when we have no choice but to introduce air into the system, for example when replacing fuel filters. The system must then be purged of the air that has entered it. In case of a problem at sea, it is important to know how to proceed on your engine, and to have identified each bleed screw to avoid staying with a drifting boat...

The diesel is sucked into the tank by a pump. It passes through filters to remove impurities. The fuel then reaches the injection pump, which pressurizes it and distributes it to the injectors, which spray the right amount into each cylinder. A return pipe returns the excess fuel to the tank, or to the filter as the case may be.

An efficient priming can only be done on a circuit in perfect condition. The slightest air leakage is unacceptable! In case of failure, follow the circuit, cloth in hand. Wipe and inspect each connection, each collar... Any greasy trace, any sign of moisture should alert you, and it is then essential to tighten or even replace the faulty fitting.

Fittings are often of the type banjo these are made up of a hollow screw that passes through a hollowed out part. On both sides, the sealing is ensured by soft metal seals, which must be replaced at the slightest doubt.

The filters contain a relatively large amount of fuel. When replacing them and to facilitate priming, it is best to fill them with clean diesel fuel before mounting them on their supports. This will prevent too much air from being forced into the rest of the system.

Every engine is different and it is important to locate the bleed nipples on your engine. These screws are identified in the maintenance manual, and you will have to open them one by one, in the order of diesel circulation, to let out all the air bubbles. Note that modern engines are often self-priming, which means that they can be bled and restarted by pressing the starter.

The priming pump is usually a simple lever placed on the diesel pump. It is often difficult to access, especially since it must be operated while purging.

First, open the bleeder on top of the filter holder. A few turns are all it takes, and operate the pump until clear, bubble-free diesel fuel flows out. You can close the drain and move on to the next one.

If the engine runs out of fuel or if you have to work on the injection pump, it will be necessary to bleed it as well. Follow the procedure described in the maintenance manual for your engine by opening the bleed screw(s) indicated in the order. On the most recent engines, they are self-priming.

"Breaking" the injectors is a mechanic's expression. It is in fact a matter of slightly opening the screws that connect them to the manifold to let the air out.

You can do this by operating the throttle, but it is tedious and sometimes unnecessary if you have an assistant to operate the ignition key. By keeping the engine slightly accelerated, this helper will operate the starter while you open one connection, then the other... Before you finish the series, the engine should start, limping a little at first, then more and more regularly.

To complete the job, and on modern engines that are equipped with a filter return bleed, all you have to do is open it while the engine is running and close it again as soon as the diesel runs clear and without air bubbles