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Old 24-02-2015, 10:31   #1
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Fuel pipe puzzler

I have a fuel line that seems to be breaking the laws of physics: the fuel return line has a "T" join in it, one line returns the fuel to the tank, while the other works as a breather, hanging well above the level of the tank. Today I was changing the filters and noticed that the bilge had a fair amount of diesel in it. The fuel, instead of returning to the tank, was being pushed out of the breather pipe. "Simple", I thought... a blockage somewhere. I have since changed all the pipes. no blockage at all, the breather is where it has always been, about 50cm above the tank. Yet the fuel still spills from the breather. I am baffled......

If this persists, does anyone know if it would be a bad idea to remove the "T", fit the return line directly to the tank and put a breather pipe directly onto the tank?

Thanks
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Old 24-02-2015, 10:47   #2
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Re: Fuel pipe puzzler

That would be a good idea. It sounds like someone used the tank vent as the return?
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Old 24-02-2015, 10:53   #3
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Re: Fuel pipe puzzler

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Originally Posted by Cheechako View Post
That would be a good idea. It sounds like someone used the tank vent as the return?
I think this is the original design, it is an old Prout and both engines have the same set up. There was one of those old pressure fuel level indicators fitted, where you pump the tanks to get a rough idea of how much fuel there is. I prefer a stick but the fitting for the old gauge could easily become a breather for the tank. Still, it is bugging me why the fuel is being pushed up ...
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Old 24-02-2015, 11:15   #4
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Re: Fuel pipe puzzler

Unless the return line to the tank is big enough that it can exchange air and fuel at the same time in opposite directions it shouldn't work in the first place. The tank is a dead end if the pipe going to it has enough fuel that air can't bubble past it at the same time.

Even if there is enough area to exchange fuel and air at the same time, the pipe running to the tank after the tee can't have any low spots. You would then pool fuel in the trap and then there would be no exchange.

Did you recently re route the pipes?
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Old 24-02-2015, 12:02   #5
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Re: Fuel pipe puzzler

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Unless the return line to the tank is big enough that it can exchange air and fuel at the same time in opposite directions it shouldn't work in the first place. The tank is a dead end if the pipe going to it has enough fuel that air can't bubble past it at the same time.

Even if there is enough area to exchange fuel and air at the same time, the pipe running to the tank after the tee can't have any low spots. You would then pool fuel in the trap and then there would be no exchange.

Did you recently re route the pipes?
No, we bought the boat a year ago and both engines had this system. I did have the same thoughts as you about the returning air but as it has worked fine up until now, I let it be. There are no low spots as the T is close to the tank end of the return line. I have not re-routed anything, the only thing that changed today was an oil change and new filters. The dead-end theory also crossed my mind (although in theory more fuel leaves the tank than is returned, so there should be a slight vacuum in the tank if anything) I let it run a while with the cap off the fuel tank: it made no difference. I put in a new line from the T to the tank in case of obstruction; again, no difference. I will be removing the T tomorrow and fitting a breather pipe directly to the tank but t all seems very twilight zone.
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