The fuel return line has a tank breather incorporated into it by using a t piece just before it enters the tank. While not ideal it has worked for many years without issue. Recently fuel started to backup in the breather. Engine runs fine , fuel return and breather clear. Opening the filler cap fixes the issue of course and there are plenty of fixes, its just confusing as why its doing it now. Breather is vertical and ends at highest point. Yanmar 3ym30 and the return flow is a steady stream back into tank. Any ideas welcome. Many thanks

check for blokage at the tank down stream from the t piece ,your set up is not ideal as a return should be direct to tank otherwise excessive tank presure will push it out the breather

Maybe the filler cap has a vent that's blocked ?

Thankyou. I hope this image helps. The t piece is only a few inches from the tank and all is clear. No second breather on the cap.

I understand where you're coming from - there's the gold standard for the air breather config, which you know about (and a hundred people will tell you to immediately change to), but there's often "other" ways that work in practice, and you want to know why your "other" way no longer works.

Question:
When the engine's running at cruising rpm, is the return flow a steady volume such that it completely fills the return hose, or is it just fuel squirts with air gaps in between ?

In my return line, I have inserted a 5cm length of clear hose so I can see what's flowing if I need to. I just hate black hose because you can't see anything, but it's also the only stuff that stays soft and seals well, so I just have a short bit of clear hose.

Maybe the engine has started returning more fuel for some reason.
could the hoses have moved position or are they secured in place? Looking at the drawing the vent would work better if it was the highest point, with the tee up the other way.

Thanks for the replies. At low revs there is a constant stream of fuel coming out of the return line into tank. It will take 30 mins before it appears backing up in the clear tube and gradually climbs up the tube at maybe 1mm per minute. I can motor for extended perriods without issue as it will take a long time to get to the top of the breather. But thats irrelevant as it shouldnt be happening. If the flow were a trickle or pulsing the breather would be able to well...breathe . so im thinking maybe there is another issue thats causing excessive fuel return. Image needs rotating 90 degs. Ie. Clear tube is vertical. Many thanks

Looking at it on the bright side - provided the engine is firing all pistons and not running lean, I would feel quite happy with a strong return flow - means the HP pump is being cooled and running well. I'd prefer that to a below-par return flow.

Though if it actually is your case that your return flow has increased, I lack the expertise to nail a cause.

Being an interior tank, how do you fill it - is there a filler hose ?

Hi 5knotsisbest

It is interesting that the tank breather has been fitted into the return line and I guess that the obvious thing to suggest would be to separate the two so that the breather can just breath air into and out of the tank and the return line can just return fuel to the tank

You said that this setup has worked for a while and if so has anything been changed recently in the fuel system and are you running the tank at around the same level you previously have or have you recently filled it for the start of the warmer weather

Also where the fuel return hose goes into the top of the tank is it to a welded spigot on the tank or to a spigot mounted into a removable plate that also has the fuel feed pickup tube as part of it

Regards Don

Thanks karsten and don. I just use a jerry can... works well enough as i dont use much fuel . Levels havent changed much and a spigot comes out from just under where the filler cap is screwed on.
Thanks again

Hi 5knots

If you don't mind me asking where in tassie are you as we are in Port Huon

Regards Don

Up in "town" don

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5knotsisbest said..
Up in "town" don

No worries

Looking at your photo I don't suppose the breather hose is kinking just above the t fitting when it is warm and stoping the tank from breathing

Also when you remove the tank cap when it is running and pushing fuel up the breather can you tell if there is pressure or vacuum in the tank

Regards Don

You may be pulling in air somewhere before the pump and returning air to the tank,, resulting in pressure in the tank and forcing the return flow up the breather

Running without the fuel cap on there is a continuous stream of fuel being returned into the tank without any air bubbles or pulsing. Fuel cap on and no probs but after running for some time i watch the fuel start to appear in the clear breather. The fuel gradually climbs up the breather. Its very slow but after an hour or so it will no doubt come out of the top of the breather. Im guessing there is pressure in the tank to cause the " backup". Open the cap and the fuel drops immediately out off breather and back to notmal. Im thinking a separate breather out of the top of the fiiler cap is inevitable but not finding the actual reason is annoying. Thanks again.

I don't suppose you can post a photo of the fuel cap upside down

Regards Don

Will do don. Now you've got me thinking...
Thanks

You should not have your breather in your return line. As the fuel heats up from the returned fuel the air expands and not allowing fuel into the tank and backs up the vent line. You should have a separate vent directly out of the tank. Do not vent the cap if there is any chance of water being near it as you will suck water as the tank cools. I actually built tanks.

Thanks jode. That makes perfect sense and aligns with my suspicions. The only reason im putting a vent in the cap is firstly i can remove the cap and put a suitable spigot in remotely rather than into the tank in situ and secondly the cap is on top of the tank under the qtr berth. I have no deck fill. Hope that makes sense. Cheers.

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5knotsisbest said..
Thanks jode. That makes perfect sense and aligns with my suspicions. The only reason im putting a vent in the cap is firstly i can remove the cap and put a suitable spigot in remotely rather than into the tank in situ and secondly the cap is on top of the tank under the qtr berth. I have no deck fill. Hope that makes sense. Cheers.

That will work then, just remember you will end up with a strong diesel smell going right through you boat, which is not very pleasant. so Just make sure you vent the cap to outside the boat which is what I think you are intending to do.

Hi 5knots
Interesting problem
You say,Recently fuel started to backup in the breather.
Does that mean all the way to the top of the breather and then spill out?

This line has got me, "l it has worked for many years without issue. "
This would indicate that something has changed.

I see that the fuel return is in the neck of the filler.
I take it that in that with the filler cap off, the fuel doesn't back up the breather hose at all.
Then when you close the filler caps it backs up.

Is there by any chance an attachment like a tube or something that has come adrift from the end of the fuel return line where it enters the filler neck? Is it threaded?

Has the fuel return line come away from a saddle or some other form of attachment and dropped lower than it should be, or conversely has it been pulled and risen higher than it was in the past?

Just ideas.

gary

Thanks all for the replies. Images attached don . Cheers.

As nothing "appears" to have changed i can only assume the issue is elsewhere and causing excessive return flow. At a low flow the return line wont be full allowing air to reach the breather. At a high flow like occurring now no air gets in resulting in pressure building up in the tank. Thanks again

Hi 5knots

Looking at the size of the hole in the fitting for the excess fuel to return through and also for the tank to breathe it is hard to see how it can do both

How much fuel is coming out of the return when the engine is running with the cap off

I don't suppose that the end of what looks like thread tape on the cap is partially blocking the return /breather when the cap is on

Also has the cap always been thread taped

This is a really interesting problem you have that I am keen to know why it is happening when you say it worked before

Regards Don

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Donk107 said..
Hi 5knots

Looking at the size of the hole in the fitting for the excess fuel to return through and also for the tank to breathe it is hard to see how it can do both

How much fuel is coming out of the return when the engine is running with the cap off

I don't suppose that the end of what looks like thread tape on the cap is partially blocking the return /breather when the cap is on

Also has the cap always been thread taped

This is a really interesting problem you have that I am keen to know why it is happening when you say it worked before

Regards Don

Hi, my thoughts as well, . Would it be possible to drill and tap another hole in a similar position slightly further around the fitting and install a barbed fitting for the exclusive use of the vent line?

Also I don't suppose that the breather/ return hose has a dip in it where it is sitting on the top of the tank and that is allowing diesel to sit in there like a water trap under a sink and stopping the breather from working

If you lift the T fitting up off the tank with the engine running and the cap on so that everything from the breather pipe is downhill will the fuel that is slowly building up in the breather hose drain away

Regards Don

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UncleBob said..

Hi, my thoughts as well, . Would it be possible to drill and tap another hole in a similar position slightly further around the fitting and install a barbed fitting for the exclusive use of the vent line?

Yep, this is what I would be doing - even go a bit larger if possible to use as the fuel return & use the smaller one for the breather. I'd also have a look around for the same size hose barb & thread & get one with a bigger bore diameter.

It looks like the nipple is intruding far enough into the throat of the filler tube to stop the filler plug, BUT I am wondering if the plug is being screwed in just far enough to partially block the flow of the return fuel as it enters, thereby creating your backup.

So try screwing the plug right in. Run the engine till there is a backup in your breather tube and then unscrew the plug a few turns at a time and see if that is the culprit.

gary

Thanks for your time don gary and bob. The tape has always been there. Ive backed the cap off a few turns also. Once I see the fuel backing up i moved the return line and breather around to avoid dips etcc... but to no avail. The stream is strong and constant. Cheers

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garymalmgren said..
It looks like the nipple is intruding far enough into the throat of the filler tube to stop the filler plug, BUT I am wondering if the plug is being screwed in just far enough to partially block the flow of the return fuel as it enters, thereby creating your backup.

So try screwing the plug right in. Run the engine till there is a backup in your breather tube and then unscrew the plug a few turns at a time and see if that is the culprit.

gary

Good point Gary - why not measure down to the top of the nipple then cut off the plug so it cannot possibly obstruct the flow. There is still plenty of thread to seal. I would also put a gasket/seal around the top of the plug & get rid of the thread tape.

if you count the threads on the cap you will see the cap does not block the return,i think the reason you did not have problems before was that your cap was leaking causd by the distortion of the fitting welds ,and the thread tape is now starting to reform the thread, you need to have a dedicated return as any tank restriction will force fuel out the breather

Good morning
I dreamed about this little problem. No answers though.
The high pressure fuel pump cannot have suddenly started to deliver more fuel to the injectors.
The pump will deliver a certain amount of fuel per revolution and that cannot change.
The injector will inject a regulated amount of fuel each stroke and that cannot change, therefore the amount of returning (unused) fuel that is sent back to the tank cannot change.
So even though it looks like a large amount of fuel, the amount returning to the tank is the same as before when there was no backup.
You have checked for a blockage or kink in the line.
So the only thing left (that I can see ) is pressurization of the tank, but if that was the case you would see bubbles rising up the bleeder hose as that pressure finds a release. The removal of the filler plug is somehow a clue.

Got me stumped.

Gary