Using the DEF tank for diesel

Someday I would like to take advantage of my Mexico unused DEF tank.

Wondering if anyone has thoughts on the following:

1) is the DEF tank material chemically able to hold diesel fuel ?

2) are there any internal pumps or anything else inside the DEF tank ?


I would think the optimal system in one in which the tanks are joined to create one large tank which would be filled by the main fill line.


If there is a hose going from the bottom of the main tank to the bottom of the DEF tank, wouldn't this be the simplest solution ? The DEF tank cap would need to allow air to escape and enter, I assume the stock cap does this.


But what would be the best way to install nipples on the tanks ?

I would think that creating a much larger tank will seriously confuse the on board computer, like remaining range. Will this create major problems other than wrong range info ?


Any input would be great..

i am going to be doing some research on this. i was wondering the EXACT same thing today.

i just purchased my MK7 so i have not had a chance to poke around.

I’ve thought about this too.

One method to connect them at the bottom is use a pair of bolt on “sumps” like the performance diesel pickup truck guys like.
You could vent the little tank into the filler neck of the larger one.

I’m not sure what vents may already exist, would need to know and understand that to start with.

Another thought is use larger connections to T the small tank filler neck into the large tank filler neck, so they both fill at same time in a fashion.
In the small tank use an electric pump (existing?) to pump fuel in a small line up the small filler neck and around the corner into the larger filler neck, through the larger T in the necks.
Advantage is no hoses on bottom of tanks, fill easily at same time, no chance of over filling while transferring, any extra overflows back to small tank, “reserve” function if only pump the fuel over when your ready, and car computer functions on main tank range and mileage are unchanged.

Any pump that would be used to fill up the def tank would need a level sensor to shut off the pump nothing like pushing diesel out the def filler neck.

I'd be warry of tapping anything into the stock diesel filler neck to create a T as I imagine the seal would be tough to keep leak free for 1,000s of miles.

5 extra gallons is pretty tempting for those looking to stretch per tank mileage.

Def tank is in a location that if you get into a fender-bender type of minor accident, it will leak....in a Jetta/Passat anyway...
That might be the biggest safety issues.
On the golf, not as much if an issue...but still.....shame to have a leak...
I might suggest you get an aux fuel pump...and a tube...run the line into your regular tank....
The just turn the pump on for a few minutes to refil you regular fuel tank on the golf....
Just remember to only run the pump when the main fuel tank is mostly empty to refil...so on a LONG trip or emergency situation, might have a use.

ZippyNH said:

Def tank is in a location that if you get into a fender-bender type of minor accident, it will leak....in a Jetta/Passat anyway...
That might be the biggest safety issues.
On the golf, not as much if an issue...but still.....shame to have a leak...
I might suggest you get an aux fuel pump...and a tube...run the line into your regular tank....
The just turn the pump on for a few minutes to refil you regular fuel tank on the golf....
Just remember to only run the pump when the main fuel tank is mostly empty to refil...so on a LONG trip or emergency situation, might have a use.

Click to expand...

Thats a good idea! Small pump that could be reversed so you can fill the "aux" fuel tank as well from the main. Flip pump on to fill main once its down to 1/4 tank to keep from any over fills.

ZippyNH said:

Def tank is in a location that if you get into a fender-bender type of minor accident, it will leak....in a Jetta/Passat anyway...
That might be the biggest safety issues.
On the golf, not as much if an issue...but still.....shame to have a leak...
I might suggest you get an aux fuel pump...and a tube...run the line into your regular tank....
The just turn the pump on for a few minutes to refil you regular fuel tank on the golf....
Just remember to only run the pump when the main fuel tank is mostly empty to refil...so on a LONG trip or emergency situation, might have a use.

Click to expand...

Another bonus is that you can add "gallons to fill" to the MFD (it's a rough guestimate), so you can watch the it go from 9 gallons to 4 gallons as it fills up.

I can't help but think that if the two tanks are combined, the on board computer will try to make sense of remaining fuel and miles traveled and fuel consumption and think

"does not compute"
"does not compute"

https://www.youtube.com/watch?v=ZBAijg5Betw


and then blow up in flames...lol

I doubt there is way to change the fuel tank capacity in vcds ?

It would just thinks its getting 90 mpg. Shouldnt hurt any thing.

I'd convert it to an auxiliary wiper fluid resevoir. No more wiper fluid range anxiety.

seriously the stock tank of the mk7 only holds like 1.5L, friggin weight savings.

Glad I'm not the only one that thinks about these things! My thoughts were the same as the OP in that some connection from the bottom of each tank would be easiest.

From what I can tell on the distance till empty. It uses the avg MPG throughout most of the tank to give a number till empty, and when the level gets to ~1/4 tank it begins to show a more true DTE. My current tune is off by 9% (~5mpg) and the DTE will show 700-800 miles at the beginning of every tank and slowly comes down to reality when it gets low which is closer to a 600 mile tank.

Same on our Alltrack, 450 miles soon turns into 350 once there's 1/3 tank left.

If you removed the original fuel tank and the DEF tank, I wonder if there would be room to put a tank from a previous generation TDI in there? Maybe from a Mk6 or even earlier.

The Mk4 has the same kind of axle beam as the Mk7 TDI (gassers have independent rear suspension) so maybe a Mk4 tank would fit. It holds something like 62 liters.

Or, the Euro TDI has axle beam setup as well without DEF? Could poke around to see if the axle beam version has a different tank part number on ETKA

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I didn't think euro Golfs run DEF.

I believe the GTD runs the IRS of the GTI, not sure about the wagens.

Yes, GTD runs with an IRS and no DEF. Which would lead me to believe that the normal Golf would also not require DEF.

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