bleeding a Yanmar SB8

zlod

zlod

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I've had to bleed the air out of my Yanmar SB8 the last two times I've started it (which is getting a bit irritating). The setup is this:

- fuel tank with no vent, going to:
- water separator filter, going to:
- lift pump going to:
- fine filter, connected by low pressure rubber hose to:
- high pressue filter, connected by metal pipe to:
- fuel injector. This is connected by a metal pipe back to the fine fuel filter

I.e. all of the return fuel goes back to the fine fuel filter.

We can get the engine going (after bleeding) and it then runs fine, but I don't get why it keeps needing bleeding.

The thing I really don't understand is that there doesn't seem to be a way of bleeding the fuel return line. *If* we get air in that, then we can go through the bleeding process, get fuel back up to the injectors and the engine will start, but we've still got that air in the return line which will presumably circulate and cause problems later (like next time we try to start the engine).

I've got loads of bits of tissue around the engine at the moment to diagnose where any fuel is being lost.

Do I need to get the air out of that return line as part of the bleeding process? If so, then how?

Any tips would be greatly appreciated.

Cheers,
Zlod
 
Ideally the fuel return would be taken right back to the tank for any air to be released and not rejoin the fuel line for another lap. I would be looking for an air leak before the lift pump because anything after the pump should be under positive pressure when the engine is running and it would have to be a fair leak to allow air to enter when the engine is stopped
 
Agree with Pete - can't see how it can be a problem with the return line. Not uncommon to have a very small leak which is too small for diesel to get out but when boat is standing, air can get in so it doesn't even show up as a diesel leak.

As a matter of interest, have any parts been disconnected because if they have been, they will need new sealing washers when reconnected. The old ones will have distorted and are unlikely to reseat properly.

So, what's the solution? As Pete says, it really is a matter of checking every connection between the tank and the injectors. It may be as simple as something needing tightened up but don't bank on it.

FWIW, what I would do is:

1. Start by checking the tightness etc of all your connections. If that doesn't work
2. In turn, take each of your fuel line components and redo the fittings.

If you decide on 2 above, I would start with the parts that are not attached to the engine as these are easier to get at - eg the water separator.

I had a very similar problem which I isolated to being the water separator by taking it out of the system by removing the pipes and connecting them with a short length of copper pipe. Bled the system and it all worked fine. Knowing that it was the separator, then it was an easy job to focus on sorting out that one part.

I appreciate however that this solution is not so easy with the lift pump or other parts that are connected to the engine. However, if you are redoing these, they should only need replacement copper washers (re comment above)

Hope this helps and good luck
 
Zlod

I've given this some more thought around the comment I have made above:

"have any parts been disconnected because if they have been, they will need new sealing washers when reconnected. The old ones will have distorted and are unlikely to re-seat properly"

Did this problem start after a service or fuel filter change? The reason I ask is that under the bleed screw on the fuel filter on my Yanmar (not a SB8 but probably similar) is one of these copper washers. Yanmar recommend that this is changed every time you undo the bleed screw due to distortion. Most people don't do this and it is often the source of a diesel leak- If fuel can get out then air can certainly get in. If you have been undoing and tightening this screw to bleed the system then the washer will almost certainly need replaced. They are not expensive and I now buy then 5 at a time for just this reason.

It may not be the answer but its easy to do and not expensive.

Again, hope this helps.
 
I have a YSB8 and have undone and done up the bleed screw on the fine filter several times and never replaced it. It is a nylon type washer. The set up is the exact same as mine with fuel return going back to the filter. I've never had a problem except after a filter change.
Did you bleed as described in the hand book? i.e. right up to the injector and take the pipe off.
If you want a PDF copy of the hand book PM me with e-mail address
 
Hi Dave and Spyro,

Thanks for your very helpful responses.

The first time we needed to bleed the system, we got into a right mess, trying to change the fuel filter. The fuel filter assembly was not in a good state. We gave up. We got the professionals in. They changed the fuel filter assembly, bled everything and things worked. We took the boat out twice and everything was OK. Then we went down one time and the engine didn't start. It was then that we bled the system (all the way to the injector, taking the top pipe out as part of the process - the one from the high pressure pump to the injector). We got the engine started and were happy. The next time we went down, we had the same issue. We bled again and got the engine started again (we're pretty pleased that we know how to bleed the system now, but it would be nice if it wasn't necessary in the first place). Since that last successful bleed, I have started the engine once more successfully (without the need for a bleed).

So the fine fuel filter assembly was removed (by the pros). I don't know whether new washers were put on or not (I assume so). I've bought a set of copper washers, so if it happens again, I might take off all the connectors and put those new washers on. There is no washer under the bleed screw. It's definitely meant to be rubber is it?

I'm hoping that I've got everything tightly done up now. We'll get to find out on Tuesday night!

One thing that we did after the last time we bled the system was to vent the fuel tank (maybe this isn't the ideal setup, but there is only a "fuel out" line and a "diesel from the fuelling pontoon in" line: unless you vent the tank, you end up with a partial vacuum in the tank - it hisses when you open the "diesel from the fuelling pontoon in" line). Maybe this has helped and fuel was getting sucked back out of the system towards that partial vacuum.

Anyway next time it looks like there's going to be a big washer replacement sesh if there is any bleeding required. If that doesn't work there's going to be a more thorough investigation of the fuel line back to the tank (including the water separator filter).

Spyro> I've got the user handbook, but I can't get a workshop manual. The lack of a workshop manual is the most likely thing to condemn the engine at some stage in the future. I was told that an SB8 is different to a YSB8 which came as a bit of a surprise to me. Is this your understanding?
 
Just re-read Dave and Spyro's posts: Dave has a copper washer and Spyro has a nylon one. Please disregard the question about rubber washers!
 
I believe the Y means it has a horizontal bore not sure about a SB8.
I have a Parts manual and a workshop manuel on electronic form I could send, if you want it PM me with e-mail address.
 
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