In spite of all my years of boating I am still surprised about the air of mystique surrounding diesel engines. On of the comments I hear a lot is that diesels are complicated to work on and not as simple as a gas engine. In fact, nothing could be further from the truth. A diesel is in fact less complicated than a gas engine, uses a less dangerous fuel, gets more mileage and does not rely on a spark to ignite the gas.
If you do have a diesel in your boat or if you are thinking of fitting one now or in the future there is one skill that every owner should learn and that is how to bleed the air from the fuel system. It is true that there are engines now on the market that self bleed but the majority do not. Learning how to bleed your diesel engine will help you understand the engine a little better and get you out of trouble without the need of resorting to a mechanic.
These basic principles apply providing a diesel is being fed with clean fuel and enough air to make the engine run. Almost without exception fuel is drawn from the tank with a lift pump attached to the engine. The fuel is first run through a primary fuel filter and water separator which is often quite large, then passes through a secondary filter sometimes fitted to the engine. From there it passes through the pump to an injector pump which squirts precisely measured amounts of fuel into the cylinders through the injector at the correct moment in the induction, compression cycle of the engine. As the pump delivers more fuel than can actually be used by the engine there will be a return pipe from the injector pump to the tank, which sends back excess fuel.
The photos listed below reference a small 1GM10 Yanmar single cylinder that I had on the bench to shoot these photos, but all engines will be similar although some of the components may look slightly different.
So here’s how to go about it
1 Assuming that the primary filter is full with fuel and has a sight bowl - drain off any water and sediment that might be sitting on the bottom of the element holder. Catch the run off into a cup, bowl or absorbent cloth.
5 Continue pumping until the bubbles stop and clean fuel starts to weep from the hole. Cummins and Volvo engines are now white which makes the diesel easy to spot as it runs out. Re-tighten the screw as you pump. The pressure from the fuel escaping will prevent any air from migrating back into the fuel line.
6 Repeat the pumping procedure only this time release the fuel delivery pipe to the injector pump half a turn then tighten as soon as fuel is ejected. The run from the fuel lift pump to the injector pump is generally short so a couple of pumps is often all that it takes. It should be noted that the fuel will squirt out with greater force because it's on the pressure side of the fuel pump.
7 Locate the other end of the fuel line where it connects to the injector. It will look something like that of the picture shown here. If you have an engine with more than one cylinder you will have to repeat the loosen, pump, tighten sequence for each injector starting with the injector closest to the injector pump. If you are unsure which this is -- it will be the injector with the shortest pipe run from the injector pump.
8. The job is done and the engine should now start. If it does not start after 10 seconds or so of cranking, double check that everything is tight. If it still will not start, try bleeding the system once more as even a tiny bit of air can prevent the fuel from getting through