connecting the fuel and DEF tanks. Discuss...

UNBROKEN · Jul 27, 2015

I hate knowing there's an extras 5 gallons of space I can use for fuel just sitting there so I'm considering marrying the two somehow.
I have a couple of thoughts but hopefully someone can expand or add some insight.
First...I think the connection would be easiest on the bottom of the tanks. A pair of sumps connected via a hose would work but would leave the sumps hanging there. The other option I looked at was using a pair of -16 bulkhead fittings on the bottom of the tanks towards the inside of the truck to protect the hose. Something like this:

So connecting them is the easy part. What I don't know is what's inside the DEF tank itself? I guess whatever pump, level sensors or whatever else would need to be stripped out? Obviously the tank would need to be cleaned extremely well, or even a new unused tank sourced? What about the tank itself...is it the same material as the fuel tank? Surely it's diesel safe?

And last...venting during filling. Is there a vent on the DEF tank or would one need to be added? If so....vent into the top of the fuel tank or add a line over to the filler neck?

Basically...I have an idea. What I'd like to discuss is whether it's worth going further or just forget about it.

CMCx2012 · Jul 27, 2015

Subscribed and interested as well!!!

CWAY6.7 · Jul 27, 2015

Ditto

BuiltPowerStrokeTough · Jul 27, 2015

Being that I just dropped the tank, emptied it, cleaned it and reinstalled it I may be able to help with a few of the questions. First, yes there is a vent tube that runs from the tank all the way up to the top of the filler neck. You can see it if you just look under the truck on the other side of DEF filler neck opening. Second, I have attached a picture of the tank out of the truck. There is a big plastic ring that is underneath the pump assembly. It simply unscrews and the pump assembly can be lifted up, but only a little ways as the pick up tub and everything is still attached underneath it. I did not remove the entire insides or even the complete pump assembly as my goal was to only drain it and clean out it. Underneath that pump, there is a big screen that is the diameter of the opening of the ring you take off and pick up tube that goes to the bottom of the tank. This screen assembly is completely open in the middle and goes down to the bottom of the tank. I know with the description its hard to understand, but I wish I would have known this was going ot be a thread posted the day after I just did the job. I would have taken more pictures to help out. Either way, it appears that the screen, pick up tube and pump would all be able to come out together. The screen assembly does move on the inside so you would be able to pull it out. Again it was not necessary for me to empty it so I did not remove it. Let me know if you have any more questions and Ill be glad to answer if I can.

Vader's Fury · Jul 27, 2015

Height of the bottom of the tanks will come into play if you use sumps. If the DEF tank is lower than the fuel tank, then all the fuel will not drain from the DEF tank into the regular fuel tank. But on the other hand, if the top of the DEF tank is lower than the top of the fuel tank, you may have an issue with fuel coming out the vent on the top of the tank, Or vise versa.

Also something to think about, unless you plan to fill the DEF tank separately from the normal fuel tank, you will need to use a large line to connect the 2. Otherwise I would think the normal fuel tank would fill up and trip the fuel nozzle before the DEF tank would be full. Therefore negating any benefit of the extra capacity of the DEF tank.

I would think that the best use for the tank would be to hook it up as a secondary/emergency tank with a small transfer pump to feed into the main tank.

UNBROKEN · Jul 27, 2015

Fill rate is why I was looking at large bulkhead fittings. -16 or -20 would be plenty.

UNBROKEN · Jul 27, 2015

The vent tube being there is a plus. I don't want to over complicate it for sure but it seems like simply connecting the bottoms with a large enough hose will do the trick...after a seriously good cleaning.