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Old 03-30-2019, 06:44 PM   #1
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Air Leak from Fuel Tank?

Still new to me, 98 Mainship Trawler with single 370 Yanmar, 2 157 gallon tanks. A couple of months ago my engine stopped leaving me stranded. Thought it was clogged fuel filters so I learned how to change them and bleed the system. A couple of weeks ago, went to start engine and it would not start, just turn over. Problem was eventually traced to air entering fuel lines and showing up in Racor bowl and eventually to FI. I have (or had) a home grown version of a fuel polisher and instead of tracking down the air leak I decided to remove and replace everything from the fuel tank valves to the input to Racor and also rebuild the Racor.
Iím still getting air bubbles in the Racor, even with a 3í hose straight from the Racor to the valve on top of tank. If I close the supply valve, bubbles stop.
All background to my real question:
What kind of fitting is on these tanks? I expected to see a pick up tube on the end of the tank fittings. Looks like a tube down a half inch or so in the tank side that I canít lift out or push down. Is it a friction fit when you tighten the fitting on to the tank and it pushes down to the tube? Went to WM but they didnít know. Also I have read the I need a anti siphon valve since my engine is below my tanks.
Help, Iím tied of of smelling diesel fuel.
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Old 03-30-2019, 07:13 PM   #2
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You sure you pulled a fuel supply line and not a fuel return or vent line ?
The fuel supply fitting should have a tube that goes to within an inch or so of the bottom of the tank.
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Old 03-30-2019, 07:35 PM   #3
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That’s what I expected to see. Definitely the supply fittings. I also pulled 1 return fitting, no tube, as expected. I tried to pull the unused supply fitting for the generator but it is being stubborn. Gen and engine were on a tee so that fitting and valve hasn’t been hooked up but I’m anxious to see that fitting and what is on end of it.
It pulls fuel up but also a bit of air.
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Old 03-30-2019, 07:42 PM   #4
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Djoub,
I don't have an answer for you, but your fuel pickup (if it enters the tank from the top) must have a tube that goes down very near the bottom of the tank. If it did not, then you would not be able to access fuel from below where the pickup tube ended (meaning you couldn't use the whole tank). My tanks feed from the bottom of the tank so therefore has no "tubes".

Do you have one of those "inspection cameras" you could put part way down the opening to get a look inside? Maybe there is a tube attached inside that is not removable?
Good luck with this and sorry I couldn't be more helpful,
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Old 03-30-2019, 07:51 PM   #5
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Quote:
Originally Posted by Djoub View Post
Thatís what I expected to see. Definitely the supply fittings. I also pulled 1 return fitting, no tube, as expected. I tried to pull the unused supply fitting for the generator but it is being stubborn. Gen and engine were on a tee so that fitting and valve hasnít been hooked up but Iím anxious to see that fitting and what is on end of it.
It pulls fuel up but also a bit of air.
If you find a fitting with a tube that goes to within an inch or so of the bottom, that is the one to use. If you are still pulling air then you likely have a bad seal between the fitting and the tube i.e. crack in the fitting, bad weld where the tube connects to the tank or something along that line.
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Old 03-30-2019, 08:05 PM   #6
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If you find a fitting with a tube that goes to within an inch or so of the bottom, that is the one to use. If you are still pulling air then you likely have a bad seal between the fitting and the tube i.e. crack in the fitting, bad weld where the tube connects to the tank or something along that line.


Or as happened to someone else the diptube developed pinholes. When the fuel level was high all was well. As the fuel level dropped the pinholes eventually were exposed. Any rough water exposed the pinholes even sooner.


Are you absolutely positive that the fitting you showed is the supply. I agree it makes little sense for no diptube indicating it is more likely a return line or vent line.

Any chance if it really is the supply that the diptube fell inside. I would not expect this since if the diptube did drop off inside then you would get NO fuel
and far more than a few bubbles.
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Old 03-30-2019, 09:06 PM   #7
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Check visually (direct or mirror) or by ďfeelingĒ with a stiff wire to confirm that you are connected to the port with the dip tube. If affirmative Iíd put my money on pinholes, or a crack in the diptube. Then you need to figure out how to put a new one in!
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Old 03-31-2019, 11:41 AM   #8
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After some thought[????] and some looking at your photos, in particular the first, of the group of three showing the elbows it looks to me like the threaded stubs are machined inside to accept a tube which would be soldered/brazed into the fitting permanently.

One might be the return which would not usually need the diptube but assuming you are correct about the other being the supply then where is the diptube?

Any sign of solder/braze. I have seen improperly done joints where the parts are improperly cleaned and heated so the filler only forms a ring at the edge of the machined recess.

The filler must fully flow into and fill the recess totally encapsulating the diptube end for a strong joint.

If that is not done and the filler only forms a ring at the edge then the joint WILL fail at some time.

Is thee ANY SIGN of solder/braze residue at the entry on the elbows?

You also mention a tee where both the generator and the engine were fed from the same line. Is that old branch to the generator blocked properly with a sealed plug? If not then maybe air can get in there.

I know people do branch engine/generators fuel feeds but there can be a price to be paid. If the line is to small one or the other, gen. or eng. may be starved when both are run. The other point is if there is any problem in the gen. fuel feed the gen. system can allow air into the engine line.
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Old 03-31-2019, 02:14 PM   #9
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Quote:
Originally Posted by Djoub View Post
Thatís what I expected to see. Definitely the supply fittings. I also pulled 1 return fitting, no tube, as expected. I tried to pull the unused supply fitting for the generator but it is being stubborn. Gen and engine were on a tee so that fitting and valve hasnít been hooked up but Iím anxious to see that fitting and what is on end of it.
It pulls fuel up but also a bit of air.
Is the gen Tee before or after the Racor? If before, does it have its own Racor? Does it have the same problem?
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Old 03-31-2019, 03:18 PM   #10
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No solder or adhesive on the fittings. The gen and engine supply are now separated, same tank different fittings. Donít know what the supply fitting to the generator is yet since I have not been able to remove the tank fitting/pick up tube.

The attached pic shows the supply(3/4Ē) and the return(1/2Ē) fittings. Coincidentally the supply (large) fitting takes a 1/2Ē PVC pipe nicely.

If you zoom in to the picture of the tank hole you can see what looks like a tube in the hole. Not the best pic but I only have a few inches clearance above tanks. I canít move the tube in the hole but I see something in there. Iím going to try grabbing it with a eazy-out next time to boat. Also going to try pushing it in with a 1/2Ē wood dowel. I also have a fiber optic scope that might help see in there.

If I canít remove it could I put in a pickup tube in what is the smaller return fitting? On one tank I also have the two unused fittings, both look like 1/2Ēthread. Could I just use these holes and put in a new pick up tube there?
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Old 03-31-2019, 03:20 PM   #11
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Still new to me, 98 Mainship Trawler with single 370 Yanmar, 2 157 gallon tanks. A couple of months ago my engine stopped leaving me stranded. Thought it was clogged fuel filters....

Our MS390 is plumbed in such a way that we can draw from one tank, or the other, or both.


You should have shut-off valves on the pick-up ports on both tanks. Try shutting off the port side tank and just drawing from the starboard. If your problem goes away, then you know that it's the port tank that is the source of the trouble.


Also, are both of your crossover hose valves open? If one or both are closed, you may have drained one tank empty and not know it, because your fuel gauge "averages" the readings of both tanks.


If one of your tanks does have pinholes in the pickup tube (which I think is unlikely in a ~20 year old boat), then you can just draw from the other tank. The crossover pipe will equalize the fuel between the tanks and you should be ok.


I assume you have checked to see that all of your hose clamps are tight...
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Old 03-31-2019, 03:46 PM   #12
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No crossover hose or fittings on this setup. All brand new hoses and clamps, all seem tight. Used Aero fuel sealent for all threaded fittings. Remember I used a short 3í hose from valve on supply fitting right to Racor, and it was sucking air so the only place is the valve, fitting on top of tank, or between the pick up tube and fitting. When I close valve no bubbles so that tells me the valve isnít leaking. The tube interface must be the leak.
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Old 03-31-2019, 06:53 PM   #13
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Just to be sure try eliminating the valve.
Brass hose adapter, 3/8 fuel hose, or to fit, directly to the Racor. Maybe even stick the hose into the tank ??
Reseal the fitting and valve, make sure they aren't cross threaded.
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Old 03-31-2019, 07:00 PM   #14
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Also no sign of sealant on threads of fitting or tank ???? I like something thick like Permatex.
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Old 03-31-2019, 07:02 PM   #15
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No crossover hose or fittings on this setup.

Really?!?! A single-engine trawler with twin saddle tanks and no connection between the tanks? Are you sure? If you have a Mainship 350/390 with a single engine, the tanks should be connected together by a hose running from the front of the port tank to the front of the stbd tank. This hose normally runs across the forward bulkhead. There are shut-off valves for this crossover at each tank.


If you don't have them, I wonder if the factory messed up and plumbed your boat for twins?
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Old 03-31-2019, 07:06 PM   #16
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No crossover hose or fittings on this setup. All brand new hoses and clamps, all seem tight. Used Aero fuel sealent for all threaded fittings. Remember I used a short 3í hose from valve on supply fitting right to Racor, and it was sucking air so the only place is the valve, fitting on top of tank, or between the pick up tube and fitting. When I close valve no bubbles so that tells me the valve isnít leaking. The tube interface must be the leak.

You have two saddle tanks and your Racor normally draws from both of them, unless you close a valve. The air is coming from the plumbing of one tank or the other, but not (likely) both.
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Old 03-31-2019, 07:13 PM   #17
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Still new to me, 98 Mainship Trawler with single 370 Yanmar, 2 157 gallon tanks...

Ok...really scratching my head here. I gotta ask...last time you serviced the filter, did you make sure to fill your racor (manually) to the brim with diesel fuel before replacing the cover? Did you doublecheck the gasket in the lid of the Racor?
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Old 03-31-2019, 07:28 PM   #18
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Really?!?! A single-engine trawler with twin saddle tanks and no connection between the tanks? This hose normally runs across the forward bulkhead. There are shut-off valves for this crossover at each tank.


If you don't have them, I wonder if the factory messed up and plumbed your boat for twins?
Iím sure. Bungs are there, at bottom and just a few inches from the front. Looks like they were welded in the tank but never connected.
PO thought the boat originally had a gas engine. Thatís another debate.
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Old 03-31-2019, 07:39 PM   #19
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Just to be sure try eliminating the valve.
Brass hose adapter, 3/8 fuel hose, or to fit, directly to the Racor. Maybe even stick the hose into the tank ??
Reseal the fitting and valve, make sure they aren't cross threaded.
You make a good point. I thought of putting a hose in a container and see what happens. Putting it thru the empty fitting hole will be easy, then just the fitting with no valve.

You also mention sealant? I read somewhere that the pickup tube may be welded to the fitting. The integrity of the seal depends on how tight you tighten the fitting which presses on the welded tube. Is sealant used to help seal the tube to fitting?
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Old 03-31-2019, 08:12 PM   #20
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Getting to the bottom of it...

Quote:
Originally Posted by Djoub View Post
Iím sure. Bungs are there, at bottom and just a few inches from the front. Looks like they were welded in the tank but never connected.
PO thought the boat originally had a gas engine. Thatís another debate.

re: "Bungs are there, at bottom and just a few inches from the front. Looks like they were welded in the tank but never connected."


Ok...that's what I guessed. So...your boat was originally plumbed for twins, but (probably) the factory effed up. Either that or an idiot previous owner removed the crossover hose, maybe because it developed a leak and he needed to slip it past a surveyor.



FYI...There is no way that any Mainship 350 or 390 boat was ever set up for a gas engine.


Now...a single diesel engine with twin saddle tanks simply MUST have the two tanks connected! Your diesel engine draws fuel from both tanks but returns unused fuel to only one tank.

This is why you must have a crossover hose.

There is no reliable way to cause a single engine to draw evenly from two discreet (un-connected) tanks, nor any way to ensure the returning fuel coming back from the engine is distributed evenly between the tanks.


So...I think we now know your problem.



The first thing you need to do is cross connect your tanks, using the bungs you identified, and with shut-off valves (per ABYC) If you send me a private message, I will send you a photo of what it should look like.


You said the bungs are there, are they capped or are the shut off valves still in place?



The good news here is that it is very unlikely that your problem is pinholes in your pickup tube(s). The bad news is that whatever idiot (previous owner?) did this....probably did a lot of other stupid stuff to your boat.


Not to worry, your new boat is lucky to have an owner who will figure this stuff out... ;-)
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