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.

[Jeff Becker]

Do you have anything on changing the actual fuel filter?

I have a Yanmar 4 cylinder marine diesel. I changed the fuel filter then bled
the system to the filter. (Did NOT bleed injectors.)
Engine starts, but now is a bit rough and I see the rpm gauge erratic. I'm wondering if I put the filter back in the cannister correctly....or if I must bleed to the injectors (4).
Any thoughts or pictures?
Thanks, Jeff

[Mark]

Jeff

On your engine there should be two filters. The actual yanmar one which is quite small and a suplementary filter, probably a Racor or some such. The filters are on the suction side of the lift pump so if there is a small air leak it may not show at the air is getting sucked in. So first thing would be to double check that the filter body is sitting into the the seating correctly. Experience has shown that this is most likely to be a problem with the smaller filter mounted directly onto the engine. Assuming that you have no probelm with the filters then bleed the injectors. Although you have a larger engine the priciple is exactly the same as shown above except that you will have to crank over the motor using the starter. When cranking it over do it with the engine stop knob pulled out or with your finger on the red stop button. Which one you have depends on the age of the engine but this is to prevent the engine from starting as you do this. Your problem is almost certainly a fuel issue and as the engine is running my best guess would be that there is a small amount of air getting in and my first target would be the small filter that i mentioned above.

[Brendan]

Hello,
Thanks for the tutorial. I have a Yanmar 3gm30f and recently replaced injectors and the high pressure fuel lines. I followed your steps in bleeding fuel on the secondary filter and at the injection pump. I spoke with a local mechanic and he said to disconnect the three fuel lines from the injectors, put paper towels down, and turn the engine over until a good stream of fuel came out. I closed the Kingston sea cock and began to turn over the engine. Once I saw a steady stream I stopped, reconnected fuel lines to injectors and she fired right up. I had her run at the dock for 45 minutes under power at 1500. When I gradually brought her down to idle she died. Never had a problem with this before. I'm thinking there is still air in the system. What do you think? Mechanic said it wasn't necessary to bleed at injectors individually, but that is what I think I need to try next...
Thank you for your time.

[Mark]

Brendan,

If your engine runns perfectly well at 1500 rpm but dies when slowed down I would suggest that this is still a fuel problem. Go back and check to make sure that you do not have air in the system although as it is running I doubt this. Is the fuel old? Try starting and running the engine from a can of fresh new fuel. If this cures the fault then you have found the problem. It could also be a clogged fuel filter. Have you changed both the filter on the engine and the primary filter? Lastly check to see that the throttle control is not retarding too much, some engines use the throttle as a kill knob. Try starting and running the engine with the throttle linkage disconnected from the governor.

[Brendan]

Mark,
Thanks for writing back with suggestions. A throttle adjustment did the trick!

[Mark]

Glad to be of help.

[Terry]

Hi. Like the others have said, thank you for the great tutorial!

I have a 1GM and had no problem bleeding everything except for the injector. I wound up backing the nut almost completely off and still no diesel from the manual pumping. Do I need to open up the throttle while doing this?

That being said, she starts up with just a pump or two on the throttle and runs great. The engine has a distinct knocking at low idle which seems to go away at 1500 RPM or so. The boat (and the 1GM) are new to me so I don't know if this is normal.

Thanks again!
Terry

[randy in early light]

Mark;
I have port and starboard fuel tanks. When I switch to the starboard tank I see air bubbles in my primary (racor) filter and the engine stalls out. The fuel tanks are new, and I manually blew air into the fuel tank vent hoses to see if they were clogged (they weren't). Any ideas?

[Mark]

Randy,
This is air getting into the fuel line ahead of the Racor. If I understand you correctly you pressurized the tank to check for leaks but none where apparent. I bet dollars to donuts that you have a very small air leak either in the fuel line itself between the tank and the Racor or on the filter itself. The thing is that the lift pump on the engine is sucking and as such is pulling air in through a small air leak somewhere. If you see bubbles in the glass bowl then it must between the tank and the filter. Pressuring the tank will not necessarily show up this leak. Check all connections, hose clamps and other parts once again. It's air on the vacuum side of the pump for sure.

[Matt Cameron]

Hi there, thanks for the article - very informative and straightforward to follow.
The other day I noticed a small amount of water accumulated in the sight glass of my engines primary filter. It was after a recent refueling and the first time I'd noticed any water in there. I drained it off but in doing so it let a stream of air bubbles in through the drain nut at the same time. It was then difficult to start the engine - about a minute of cranking before it reluctantly started to fire.
Is this normal? Should the water drain introduce air into the fuel system?
Any advice would be appreciated.
Regards, Matt

[mark]

Matt

The had starting is due to air being introduced into the system after you drained the filter bowl. You should not have to crank he engine for that amount of time, it's hard on the engine and you risk filling up the exhaust with water as the pump is running as the engine cranks. In extreme cases can back fill the exhaust up to a point where water flows back into the cylinders causing a hydraulic lock. It sounds like you need to bleed the system as I outlined in the feature and then you should have no more trouble.

[Svend Aage Madsen]

Can I start my 1gm10 directly somewhere on the motor? There is a long way from starter in the cockpit to the motor in my boat. It helps when I activate the decompression handle while trying to start in the cold months.
Best regards.

[mark]

I assume that the reason that you want to crank the engine while close to the engine is to prime the pump for bleeding the diesel. Yanmar make a remote starter that clips onto the starter solenoid. I found this one on line http://www.summitracing.com/parts/anm-cp7853?seid=srese1&gclid=COCbxODTgcICFUsV7AodjHUAnQ which will work fine and costs less than $20. If you are opening the decompression on a regular basis to help start the engine then it is usually indicative of a problem with the engine as you should not have to do this.

[jim hatch]

I put a new fuel filter on my 1gm Yanmar diesel engine, but cannot get fuel from the tank to fill it up while using the hand pump on the engine. The glass bowl does not unscrew from the filter bowl, so should I put the filter in a bucket of diesel to fill it and then reinstall the in and out lines?
Or is it simply an air block preventing lifting of the fuel?