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Old 03-29-2018, 03:15 PM   #1
Ruffus
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Auxiliary fuel tank HELP!

I added a gravity flow RDS 45 gallon vertical auxiliary fuel tank in my 2016 F-350 dually. The connections to the filler hose have no leaks but it appears that I have fuel leaking from the top of the stock fuel tank, any ideas what might be causing this?
Thanks in advance
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Old 03-29-2018, 03:57 PM   #2
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Yes, my friend in Phoenix has a 2016 F-350 and his leaked also at the top of his Ford tank. His neighbor is a Dodge mechanic, they dropped the stock tank and sealed the leak. That was more than a year ago and it hasn't leaked since.
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Old 03-29-2018, 04:09 PM   #3
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Assuming it didn't leak before you installed the second tank, I'd first check to make sure the plumbing for the new tank is secure where it enters the OEM fuel system. If that is not the source of the leak, then, since it's a gravity fill system, it sounds like the factory fuel tank is overfilling and the excess is leaking out "somewhere". I'd check the fuel cap to make sure it's not leaking both from the cap and from where the filler pipe attaches behind the bed sidewall (there are three bolts that hold the hose to the back side of where the cap screws into the filler pipe). Then I'd check all the clamps on the overflow hose. Here's a picture of the filler pipe. As you can see, there are 3 bolts holding the top in place and several clamps (either 4 or 5, depending on version of the filler pipe) that secure the hose to the tank.

Most of the above is fairly easy to access with a creeper and some smooth concrete to roll around under the truck. There's almost enough room to roll over under there, and it's fairly open around the fuel tank.
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Old 03-29-2018, 05:54 PM   #4
brad2388
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Are you using some kind of valve to shut the aux tank off when not needed?

The cap is vented and will leak if the tank is overfilled.


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Old 03-29-2018, 07:07 PM   #5
Barbell
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I put the same tank on my '16 3500 dually. Often leave the gravity feed valve open when the main tank is full w/o any leaks so far. The line from the aux tank taps into the filler pipe with a tee cut in. Only place I see it could leak assuming all connections are tight is around the filler cap. No leaks after six+months.
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Old 03-30-2018, 01:15 AM   #6
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What Brad said is not so. It is not vented where any fluid will leak OUT, only air would come in, as in a vacuum check valve. I've had this exact tank for more than three years and no leaks. And it is never turned off at the tank.
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Old 03-30-2018, 02:26 AM   #7
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It won't fix your leak but if you install a normally closed 12v valve in your 3/8" line, you can remotely shut off your fuel draining into your stock tank and prevent the leak and only turn on when wanting to add fuel. I plan on doing that on my Ram, not expensive and easy to install and never any worries about fuel leaking or overflowing the tank.
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Old 03-30-2018, 07:32 AM   #8
E Rod
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Quote:
Originally Posted by Ruffus View Post
I added a gravity flow RDS 45 gallon vertical auxiliary fuel tank in my 2016 F-350 dually. The connections to the filler hose have no leaks but it appears that I have fuel leaking from the top of the stock fuel tank, any ideas what might be causing this?
Thanks in advance
I have the same tank but a 60 gallon and have no issues. Make sure the auto shut off is facing up and that all clamps are tight. I installed clamps purchased at NAPA. Better quality
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Old 03-30-2018, 09:33 AM   #9
travelin texans
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Always knew Ford's used a lot of fuel, but never heard they made so much the tanks would run over.
My trucks stock tank hold 36 gallons, once down to about a 1/4 is about all I want to drive (300+/- miles) a day anyway.
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Old 04-08-2018, 04:57 AM   #10
roadglide
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Another problem but 2018 gmc duramax. I have RDB 50 gallon tank it's set with valve filters and pump and switch inside the cab. The fuel tank is pressuring up. The pump slows down when I loosen the cap air is released . The air vent hose going to the top fuel tank has some kind of valves secured to the fuel tank. . I had the RDB tank on 13 duramax and it spill diesel if left on .
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Old 08-13-2019, 10:47 AM   #11
X-Jaywalker
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Is it possible the vented gas cap is bad or vent tube clogged or???. Causing elevated pressure through the system? I have a tenancy to over think stuff.
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Old 08-13-2019, 12:36 PM   #12
wiredgeorge
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Quote:
Originally Posted by notanlines View Post
What Brad said is not so. It is not vented where any fluid will leak OUT, only air would come in, as in a vacuum check valve. I've had this exact tank for more than three years and no leaks. And it is never turned off at the tank.
Motorcycle mechanic here... venting equalizes air pressure inside and outside and the vent should NOT leak fuel IF the tank is NOT filled to the top on a hot day. On a hot day, fuel will expand in the tank and come out the vent if the tank is topped off. Think the spare tank in question will be the same. I do motorcycle carburetors and if a bowl vent is capped off, the carb floats go nuts and puke gas or won't allow flow. They sure won't work right with the vents plugged.
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Old 08-17-2019, 11:51 AM   #13
CaptnJohn
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On my truck expansion is created in the truck tank, not the auxiliary, fuel is forced out the truck tank and drains from the auxiliary. For that reason only a gravity auxiliary tank is all Ill ever have.
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Old 08-17-2019, 02:47 PM   #14
LewisB
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I'm in the same approx camp as wiredgeorge. We have a 90 gal aux tank and a 48 gal truck tank. If I leave the aux tank valve open, the truck tank fills to the top but doesn't seem to overfill - the fill cap is not vented as notanlines pointed out. However, if the truck tank is totally full, the aux tank drain valve is closed, the truck is just sitting, and the daily temperature gets very hot (say 110F), the system will build pressure from the heated fuel in the full truck tank and begin to "drip". I have always attributed this to fuel expansion in the system with no place for the pressure to vent other than out of the system.

This never happened before installing the aux tank because it just was not possible to completely fill the truck tank and fill lines all the way to the cap using normal fuelling procedures. Once the aux tank was added, it drains into the fuel fill line and it becomes possible to completely top this system off with fuel.

Once I had this experience, I started either leaving the aux drain valve open (so that pressure could push back into the aux tank) or closing the aux tank valve and making sure that the truck tank was not totally full. This gives the pressure some place/way to vent.

Not sure if that is exactly what happens, but it seems to work for me.
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