Auxiliary Tank Use

[axel64]

Hi all,

I just purchased a 1999 HZJ75 with a 140L auxiliary tank. I just don't understand how it works and seek some help.

The seller told me that there is a transfer pump with which I can transfer fuel from the 140L auxiliary tank to the 90L main tank. The pump worked when I purchased the car, but the 2. tank was empty. So I filled up the main tank and used it. A couple days later I filled up both tanks and the vehicle used the fuel from the auxiliary tank only - without me using the transfer pump at all. So I am wondering what kind of setup this is and what happens when the auxiliary tank gets empty. Will the main tank automatically be used and if so, how. As I said in the beginning: the main tank was used when the auxiliary was empty. But how can it be that only the auxiliary tank is used when both tanks are full? I dont understand this. Can anyone give me their two cents worth, please?

Cheers, Axel.

 

[Bmonck]

That seems an odd set up. unless aux tank automatically pumps into the main tank. Then it would look like you aux tank is going down ie straight to the engine but it is really going to main tank then the engine. So then main tank looks continuously full Until aux tank is empty.
I have two tanks but have to push a “sub” button to use that second tank. It then pumps directly to the engine not via the primary tank

 

[axel64]

That seems an odd set up. unless aux tank automatically pumps into the main tank.

Yes, this is my guess as well. Would be happy to hear from anyone who has a setup like this, where the fuel is automatically pumped from the aux into the main tank. Will see what happens when the aux gets empty.

 

[stumog]

I'd be looking for a switch on the dash or a valve under the bonnet to work out how its switched as I very much doubt its automatic

 

[axel64]

I'd be looking for a switch on the dash or a valve under the bonnet to work out how its switched as I very much doubt its automatic

Yes, I have a switch under the dash but I have not used it, nor manually turned it on. Still, my aux tank is getting emptied as I drive, while the main tank remains full.

 

[stumog]

Try switching it over and using see if the fuel gauge drops on the main tank. Should see a change within 100 miles

 

[Gr8Yota]

Sounds like fuel is being gravity fed into the main tank. So whilst you are using fuel from your main tank it is being fed constantly hence you gauge readings. This is how the (OE) FJ aux tank works. If your tank has a transfer pump maybe the valve is stuck open and it is just pulling fuel through.

 

[GeekOKent]

Is there a story around fuel return as well?

 

[axel64]

Sounds like fuel is being gravity fed into the main tank.

This seems to be the case, yes. The pump is only to transfer the remaining fuel from the aux tank that is not gravity pulled (if necessary).