Bleeding a diesel engine

    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.

2 Locate the lift pump that will be attached to the engine somewhere, look in the owner’s manual if it is not apparent but you can often find it by tracing the route of the fuel line.

3 Many lift pumps have a small lever that can be manually operated with a finger, shown here marked in yellow paint.

4 With a suitable wrench open the bleed screw on top of the secondary filter as you operate the pump. You only need to loosen the screw.  Do not remove it completely.

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

Tip:  It is a good idea to paint all the bleed points with paint that is a contrasting color to the rest of the engine.  This will help you to easily locate them.

Comments

[Grant Williams]

Nice to see your excellent photo's. I have a 1GM10 Yanmar, just wished mine looked that clean! I had to bleed mine after a filter change and as I was new to this and feeling a little intimidated I checked on the net for information Ifound some results but some had omitted vital information as I later discovered. I went to a marine engineer and he said to bleed it to the pump and I asked what about the injector and his answer was that the injector acts like a one way valve so no need to bleed it there. I finally found a book in the local library by Nigel Calder and he states that you have to bleed at the injector as well with the throttle in full position,something to do with fuel delivery. Also noted in the Yanmar book it states that if the lift pump doesn't pump rotate the engine 180 degrees as to allow the lift pump to sit in the correct position in relation to the camshaft lobe. I wish Yanmar made changing the engine filter easier as you have to place the filter in the bowl as you put it back.
Grant

[adam]

I would like to know about problems with the water pump on the Yanmar 1gm10 8hp and a way to stop it leaking. I've been through 6 shafts and 12 oil seals and I still have the same problem of water leaking.

[AJ]

Thanks so much for this. As it happens, I have 1gm10 so these pictures and these steps were EXACTLY what I needed! Very helpful.

[mark]

Glad it worked out for you. Thankfully I was able to shoot these pictures on a new engine before it was installed in a boat.

[Simon Judge]

This was a very helpful article.

We have a strange and intermittent fuel problem with our 4 cyl diesel (fitted tank -> large filter -> electric pump -> injector pump). What I don't quite understand is how one can bleed the filter when (as in your article) it is on the inlet side of the lift pump - surely it then operates under negative pressure, and so if one opens the bleed point one would suck air in rather than blow fuel out?

We have had some discssion with the boatyard about the relative position of the pump and the large filter. (there is a small paper filter in the pump itself)

[Mark]

Thanks for the encouragement. I looked at your website and it looks to me as though you have a Perkins 4-107 or 108 in your boat. Anyhow the methods for bleeding are as i described even given that your engine is larger. You will see that I drained off any water from the filter bowl then used the lift pump to pull fuel from the tank through the filter. Providing that there are no air leaks on the lift side of the pump then the top of the on engine filter, injector pump and injectors are all on the pressure side so pumping the fuel primer will force air and ultimately fuel from the system. If there is an air leak on the vacuum side of the pump or a hose is loose or whatever then you can pump until the cows come home and you will only be sucking air into the fuel line and it will be impossible to bleed the system. Please let me know if this is not helpful and I will try to give you some further advice.

[Simon Judge]

Mark

Thanks - all is becoming clearer. The way the plumbing is set up at the moment it goes from the tank to the on-engine filter, to the fuel pump, and then to the injector pump.

Given the yard say the fuel is full of water (they say they have pumped out 8 litres already!!) I think we need to rearrange so it goes through a primary filter (or just a simple water trap), then to the pump, then to the on-engine filter, and then to the injector pump. As you say, the on-engine filter can then be bled.

[Phil]

Thanks so much for your no nonsense step by step approach. I made several novice mistakes when I bled our new Yanmar 3jh. I changed the fuel filters & omitted to fill them with fuel prior to installing them - full of air that had to be bled out of the system. When bleeding the injectors I pulled the line completely off the injector 1"/ 25mm, & turned over the motor until fuel pumped out. Experienced person tells me only need to just loosen the nut and close it while spurting fuel so minimal chance of getting fuel back in. (Still won't start?!). I will try again today. Thanks Lucky in Cairns Australia.

[Steve Bailey]

Hi Mark,

Excellent article! I have a question though. When one gets to step 7 where you bleed air from the fuel line where it connects to the injector, you say to "repeat the loosen, pump, tighten sequence". Two books I have read say to crank the engine to bleed the air at this point. Will pumping the lift pump manually (as you say) do the same thing?

Thanks,

Steve

[Mark]

Steve

Thanks for the message. The short answer is that yes it will. You can crank the engine and this is something that has to be done on larger engines without a manual pump on the side of the pump body. I prefer to do it manually however if possble as it prevents diesel from squirting out everywhere and plus everything is a little more controlled. You only need to undo the injector nut a half to three quarters of a turn at most.

[Diesel mechanic]

Very nicely described diesel engine repair. This is very useful information regarding diesel engine repair, if we can get help online then there would not be much need for the diesel mechanic.