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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 Iíll 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 110įF), 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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