Strange Fuel System Behavior

[Dockhead]

I changed the fuel filter today on my Yanmar 4JH3 HTE. I put a funnel and empty 5 liter oil container under the old one and opened the valve on the bottom to drain it as usual. It drained and drained . . . and drained and drained . . . WTF? The little filter only holds 200 or 300ml at most.

Is it some flow from the generator return line? (The genset was running). Shut it off -- no difference. Shut off the fuel cock. No difference.

So I pulled the filter off altogether, but the fuel kept coming! Got the new one on and screwed on, then set about cleaning up the mess in the bilge.

Why? Where did it come from? By the time it was all over, the 5 liter oil container was almost full! I poured it back in the tank of course (5 liters of fuel!).


After that, I made an experiment based on something read on here some months ago. Instead of bleeding the system as I always do after a filter change, I just pumped the hand pump on the filter boss and started up the engine. It ran as if nothing had ever been done to it -- didn't miss a beat. I didn't bleed anything. I'll be d****d, all those years of bleeding it for nothing

[a64pilot]

Sounds like your vent may be clogged and you have a little positive pressure in your fuel tank?
Warm fuel as in return fuel from the generator could slightly pressurize the tank. Of course I'm assuming your tank is lower than your filter so it couldn't be a simple siphon.

On edit, some engines can "eat" a little air, it just goes back in the fuel return line, yours is apparently one of those that can, actually I think most "modern" Diesels can.


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[Dockhead]

Originally Posted by a64pilot

Sounds like your vent may be clogged and you have a little positive pressure in your fuel tank?
Warm fuel as in return fuel from the generator could slightly pressurize the tank. Of course I'm assuming your tank is lower than your filter so it couldn't be a simple siphon.

On edit, some engines can "eat" a little air, it just goes back in the fuel return line, yours is apparently one of those that can, actually I think most "modern" Diesels can.


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Thanks -- I think I now understand exactly what happened.

Completely full tank, so level of fuel in the tank is higher than the filter. That's the answer I think.

[FSMike]

Originally Posted by Dockhead

----Completely full tank, so level of fuel in the tank is higher than the filter. That's the answer I think.

If that was the problem, how come shutting off the fuel cock didn't stop it?

[a64pilot]

Originally Posted by Dockhead

Thanks -- I think I now understand exactly what happened.



Completely full tank, so level of fuel in the tank is higher than the filter. That's the answer I think.

I was going to ask if it was possible that the fuel was all the way up the filler neck, but then though nah, not with the genny running, plus could your filler tube hold 5 L?
It was either that or a clogged vent, or gravity doesn't work where you are
I guess there is one more possibility, a lift pump before the filter, but that is usually a LOT of fuel coming out.


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[a64pilot]

Originally Posted by FSMike

If that was the problem, how come shutting off the fuel cock didn't stop it?

Good point, maybe coming back up the fuel return line?


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[Dockhead]

Originally Posted by FSMike

If that was the problem, how come shutting off the fuel cock didn't stop it?

It must not have been really off. Only explanation. I only guessed at an "off" position for the lever on the dual Racors and must have guessed wrong.

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[a64pilot]

I think the opposite of off is both on the dual's, I was a little surprised to find there was a both selection.


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[S/V Alchemy]

Originally Posted by Dockhead

Thanks -- I think I now understand exactly what happened.

Completely full tank, so level of fuel in the tank is higher than the filter. That's the answer I think.

So the flaw is actually a feature: you have gravity feed!

The point about the potentially occluded vent is a good one, however. I just had to check this to solve an issue, and have had the dreaded "runs 20 minutes, dies, wait 20 minutes, and she runs"...which was a vent line issue in the ending.

[tinkrman69]

you said you shut off the fuel cock. is it betwixt the tank and filter - where it should be?

if the fuel still flowed, after shutting off the cock I'd check that cock. If its back flowing from the engine side then you have some fuel lines routed wrong. Fuel should not be able to flow back through the return lines, your tank should not be able to fill to the top if so the vent is in the wrong place, as there should always be air space in any fuel tanks, no if ands or buts. liquid does not compress, so air space is needed for expansion and contraction due to heating and cooling of the environment the tank is in.

[Sun and Moon]

Originally Posted by Dockhead

I changed the fuel filter today on my Yanmar 4JH3 HTE. I put a funnel and empty 5 liter oil container under the old one and opened the valve on the bottom to drain it as usual. It drained and drained . . . and drained and drained . . . WTF? The little filter only holds 200 or 300ml at most.

Is it some flow from the generator return line? (The genset was running). Shut it off -- no difference. Shut off the fuel cock. No difference.

So I pulled the filter off altogether, but the fuel kept coming! Got the new one on and screwed on, then set about cleaning up the mess in the bilge.

Why? Where did it come from? By the time it was all over, the 5 liter oil container was almost full! I poured it back in the tank of course (5 liters of fuel!).

Dockhead, there is a certain balance to the universe that is always maintained. We all know of our boats' ability to consume money. Where do you think all that money goes? It cannot simply disappear - it must manifest as something, somewhere, sometime. Do not question the wisdom of the creator. Just collect that diesel and put it back in your tank, sell it or give some to those less fortunate than you. If times get hard, know you have a source of wealth to rely on.

[Ericson38]

When I worked for Exxon Marine in the 70s, we had such a thing as run away engines (GM 71 series for instance). Bottom line was to know where the fuel cock was so you could shut it down.

Do you have a distro manifold that takes a line from the tank, and then with valves, you open and close various branch circuits ? This is before the filters. I don't know if this failure mode is even possible with a Yanmar.

[jreiter190]

Don't know what the problem was but, ain't it heat to have a diesel that will start up without having to bleed it? I've got a little 3 cylinder Kubota and I love it. A little over 300 pounds and 28 ponies.

[Dockhead]

Originally Posted by Ericson38

When I worked for Exxon Marine in the 70s, we had such a thing as run away engines (GM 71 series for instance). Bottom line was to know where the fuel cock was so you could shut it down.

Do you have a distro manifold that takes a line from the tank, and then with valves, you open and close various branch circuits ? This is before the filters. I don't know if this failure mode is even possible with a Yanmar.

I have cable-operated emergency shut off fuel cocks with the handles in the lazarette, but obviously couldn't reach them when I was in the engine compartment with hands full of filters.

There's also a SeaFire system which with solenoid fuel cocks.

But the only thing I could reach was the dual Racors. I am absolutely sure that what I thought was "off" -- handle up and pointer down -- was actually "both". Interestingly, there is nothing about the meaning of the handle positions in the Racor manual . . .

[Dockhead]

Originally Posted by Sun and Moon

Dockhead, there is a certain balance to the universe that is always maintained. We all know of our boats' ability to consume money. Where do you think all that money goes? It cannot simply disappear - it must manifest as something, somewhere, sometime. Do not question the wisdom of the creator. Just collect that diesel and put it back in your tank, sell it or give some to those less fortunate than you. If times get hard, know you have a source of wealth to rely on.

Tee hee. Enjoyed that one