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I have a 2017 3500 dully and I want to install an auxiliary fuel tank because I pull a 5th wheel. I have an RDS tank and install kit from Northern Tool and have a question about the shutoff valve. Do I leave it open or closed when driving? What keeps it from overflowing when the main tank is full? The instructions also say that some but not all trucks have an open vent on top of the main tank and to run a hose from this vent to the height of the auxiliary tank. Is this to keep it from overflowing or what? Any help is appreciated.
 

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2011 Ram 2500 Big Horn Crew/Short, H&S MiniMaxx w/Trans Tune, 5" TBE w/Muffler, EGR left in place
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Respectfully, it sounds like you should have someone install this for you. Fuel and evaporative systems aren't something to experiment with in my eyes.
 

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Closed when driving, if it's a manual fill. Only open when you're parked then refill your tank.

If you want your system to be connected, it cannot be a gravity fed system. That's per DOT i think.

check out transferflow website for ideas. They have an electronic control that automatically fills from aux to reg tank.

How many extra gallons do you need? There are extended size tanks that fit under. Pricey but no issues.

I have a 110 aux tank in the bed but I have a pump and manual filler.
 

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I just checked out northern tool. It doesn't say open or closed. But does suggest you can keep open. But if you look at the diagram(and I totally think they did that on purpose) the top of the tank is below the fill cap. If that's the case it wont over flow. But if your fuel level in the aux tank ever gets above the top of the fill neck...
 

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The tank's vent is below the level of the bed on my 3500. Not that it would be hard to extend the line.
 

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The tank's vent is below the level of the bed on my 3500. Not that it would be hard to extend the line.
Really, I do not know if fuel will spill out even in fuel level is over the fill neck when fuel cap is on. But I'm pretty sure lets say your tank is full and your aux tank is full and manual valve is open and you decide to pop off that fuel cap... it'll flow out.

But OP is asking, I think, will fuel spill if both tanks are full and you have your fuel cap on...

So far i've searched and all the manufacturers that have an electronic automatic fill system only sells them with their tank. I do not see a "replacement" electronic automatic fill system - aux tank by itself.
 

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Tractor supply has aux. tanks , pumps. pump handles and what not .
 

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I have a 2017 3500 dully and I want to install an auxiliary fuel tank because I pull a 5th wheel. I have an RDS tank and install kit from Northern Tool and have a question about the shutoff valve. Do I leave it open or closed when driving? What keeps it from overflowing when the main tank is full? The instructions also say that some but not all trucks have an open vent on top of the main tank and to run a hose from this vent to the height of the auxiliary tank. Is this to keep it from overflowing or what? Any help is appreciated.


I have an RDS tank in my 2017 3500. I leave the shut off valve open when driving, and close it when I’m parking it for awhile. There is a check ball in the fitting that you install in the filler neck, that closes once the fuel level reaches that point, to prevent over filling. And I did not mess with the factory ventilation.


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Closed when driving, if it's a manual fill. Only open when you're parked then refill your tank.

If you want your system to be connected, it cannot be a gravity fed system. That's per DOT i think.

check out transferflow website for ideas. They have an electronic control that automatically fills from aux to reg tank.

How many extra gallons do you need? There are extended size tanks that fit under. Pricey but no issues.

I have a 110 aux tank in the bed but I have a pump and manual filler.


Gravity feed is NOT illegal in this situation.


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The tank's vent is below the level of the bed on my 3500. Not that it would be hard to extend the line.
I lied. Or at best, I forgot.
The vent on a pickup is at least partway up the filler hoses, but when I removed the bed it ended up not much above the frame rails.

Now I'm curious where it is on a cab and chassis, where nothing is above the frame rails, except for the filler neck.
 

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Mine will leak out the top of tank. I will only leave it open when driving and the tank is below half.

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