Air Leak from Fuel Tank?

[Djoub]

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.
I’m still getting air bubbles in the Racor, even with a 3’ 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 can’t 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 didn’t know. Also I have read the I need a anti siphon valve since my engine is below my tanks.
Help, I’m tied of of smelling diesel fuel.

 

[boatpoker]

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.

 

[Djoub]

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.

 

[firehoser75]

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,
Tom

 

[boatpoker]

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.

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.

 

[C lectric]

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.

 

[Island Cessna]

Check visually (direct or mirror) or by “feeling” with a stiff wire to confirm that you are connected to the port with the dip tube. If affirmative I’d put my money on pinholes, or a crack in the diptube. Then you need to figure out how to put a new one in!

 

[C lectric]

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.

 

[koliver]

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.

Is the gen Tee before or after the Racor? If before, does it have its own Racor? Does it have the same problem?

 

[Djoub]

No solder or adhesive on the fittings. The gen and engine supply are now separated, same tank different fittings. Don’t 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(3/4”) and the return(1/2”) fittings. Coincidentally the supply (large) fitting takes a 1/2” 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 can’t move the tube in the hole but I see something in there. I’m going to try grabbing it with a eazy-out next time to boat. Also going to try pushing it in with a 1/2” wood dowel. I also have a fiber optic scope that might help see in there.

If I can’t 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 1/2”thread. Could I just use these holes and put in a new pick up tube there?

 

[Riverguy]

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...

 

[Djoub]

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 3’ 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 isn’t leaking. The tube interface must be the leak.

 

[Solly]

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.

 

[Solly]

Also no sign of sealant on threads of fitting or tank ???? I like something thick like Permatex.

 

[Riverguy]

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?

 

[Riverguy]

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 3’ 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 isn’t 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.

 

[Riverguy]

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?

 

[Djoub]

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?

I’m 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. That’s another debate.

 

[Djoub]

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?

 

[Riverguy]

Getting to the bottom of it...

I’m 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. That’s 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... ;-)

 

[Riverguy]

So, maybe your pickup tube fell out of the fitting?


These are $20 on Amazon...

 

[Djoub]

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.

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.

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.

So here is the latest chapter, not resolved yet but I think the problem has been identifed.
First picture is of the crossover bung which has never been connected. To control flow, the tanks each have valves for supply and return. In normal configuration you can withdraw from a tank and return to same tank, or return to the other tank (not recommended) if you need some balancing. Nice feature to keep the tanks isolated if you want to or need to for any reason.

Now for the problem; I called the manufacturer of the tank, Florida Marine Tank of North Carolina. They were very helpful and actually pulled up the drawings of the tanks. After 22 years they still have the drawings on file! They told me that the pickup tube is welded to the bottom of the threaded bung before it goes in the tank and then the PU/bung assembly gets welded on to the top of the tank. See picture from their website. That is why I could not move it or remove it. They said that the weld or the fitting to the bung may have developed a pin hole and allowing air to be pulled into the line. This makes sense since when I close the tank valves the air bubbles go away. If it was leaking anywhere else it would draw more air into the system because of the higher vacume.

Now for the really good news; They make a replacement (nylon ?) pick up tube that fits inside the current welded tube. The new tube has a 90 degree aluminum fitting on top along with a anti-siphon valve all for $60 bucks. Only problem I see is because it is a semi-rigid tube I may need to cut an access hole in the floor above the fitting to get it in the hole. Not a problem as a sofa sits over the potential access hole.

New tube should be here today and hopefully I will be able install it over the weekend. If it works, I will replace the other tank assembly also.

Thanks for everyone’s suggestions and help. Will post a final follow up after everything is installed and working (fingers crossed).

 

[psneeld]

And whether you need a crossover is debatable.



In some cases necessary or desireable...some cases not needed. My boat and many I have run does/did it have one.

 

[Island Cessna]

This thread reminds me of one on my “pet peeves” from my days in industrial maintenance. Electricians were the worst, and since no electricity is involved in this thread I’ll stick with them to minimize offending all the contributors of good ideas in this thread.
Something quits working, and the electrician is called. He does his troubleshooting and decrees that “it is wired completely wrong, no wonder it doesn’t work”. Boss tearing hair out “it has worked for 20 years this way, something changed that made it stop working. Go find what changed”.
I fall into the trap all the time. Fixing things most often requires finding what changed, and I go off critiquing and looking for improvements in the whole system. The latter is a worthwhile endeavor, but if the goal is to restore yesterday’s operating condition, it sometimes delays me finding “what changed”, and getting going again.

 

[Island Cessna]

And whether you need a crossover is debatable.



In some cases necessary or desireable...some cases not needed. My boat and many I have run does/did it have one.

Don’t forget that if both feed valves are open, and there is no air in the piping the top feed lines from both tanks are in fact a “crossover”, and will equalize the tanks as a siphon just as surely as a bottom connected crossover. You do in fact have a “crossover”, with the advantage that instead of draining both tanks into the bilge everything will stop if something in the crossover piping breaks.

 

[Solly]

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?

Sealant was for any threads on the fittings. No way to seal the tube ( inside the tank) to the fitting but everywhere else there are threads should have sealant.

The idea with the hose is to reduce the number of connections (and possible leaks) to a minimum.


Interesting that the mfg. makes a replacement part... Just wondering if the problem has showed up B-4... Hnmmmmm.

 

[psneeld]

Don’t forget that if both feed valves are open, and there is no air in the piping the top feed lines from both tanks are in fact a “crossover”, and will equalize the tanks as a siphon just as surely as a bottom connected crossover. You do in fact have a “crossover”, with the advantage that instead of draining both tanks into the bilge everything will stop if something in the crossover piping breaks.

Even with both feeds open, my tanks don't equalize as in a true crossover. They may a little sometimes, but I don't expect them too.

 

[Steve DAntonio]

re:

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.

I'm afraid this is incorrect. I have built boats with twin saddle tanks, single engine, no cross over. After the boat is built the supply and return valves are adjusted until tanks are drawn from and returned to evenly, after which the handles are removed and wire tied adjacent to the valve. It can be made to work properly.

A balancing line definitely makes life easier, but some believe that a balancing hose, plumbed to the bottom of tanks, is a liability, as a leak means a very big spill. There is no ABYC prohibition against balancing lines or fittings on the bottoms of fuel tanks. Because it's readily available in this diameter, the hose used for these lines is typically A2, which is technically rated for fuel fills, unlike A1 it is not rated for continuous immersion in fuel.

As a test, as others have suggested, I'd drop a clean hose into the tank as the test pick up. I have seem metallic and Nylon pick up tube break and fall into he tank.

 

[Noahstvr]

@Djoub,
Glad to see that you are tracking down your fuel feed issue. One thing I do notice, the red plugs in the bungs for the crossover connection look suspect to me. I would consider either replacing them with threaded aluminum bung plugs or install the shut off valves and crossover hose. The hollow red plastic plugs currently installed look like the temporary plugs that are in place when the new tanks are shipped from the manufacturer. If that is the case, I would be replacing them sooner than later, thanks,
Gary

 

[Jeff F]

A single diesel engine boat with twin saddle tanks MUST have the tanks cross-connected. Think it through and it'll become apparent.

I too am going to disagree on this. I have a single diesel with saddle tanks, and valves for both supply and return. No dedicated cross connection. I always return to the same tank as I draw from, and manage athwartship trim by drawing from the appropriate tank, switching back and forth to draw them down somewhat evenly. Usually I switch every couple of days. I also have water tanks P&S, and so if one is full and the other empty I can maintain trim by running one fuel tank at a higher level to offset the imbalance. This system has worked well for me.

If the fuel return goes to only one tank then a cross connection is needed, but that seems to me to be a short cut solution.