Strange problem with dual fuel tanks

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Mar 11, 2014
Hanover, Virginia
So, this was crazy. 1977 FJ40. I replaced my front fuel tank with the one sold by Cool Cruisers. The one that is listed as a clone of the original. It fits great. I also replaced the fuel cap with one they sold. All seemed fine. I had five gallons of gas in a can, so dumped it into the new tank. So, I am driving in to town (using the front tank) and I notice the gas gauge keeps going up. Seemed nutty. When I stop I remove the gas cap and the tank is totally full of fuel. All the way to the cap. I figure it came from the rear tank some how.

Yesterday I got under and removed the fuel tank switching solenoid. Checked it and it switches correctly. I blow into the inlets and when switched, the expected inlets are what is passing air when I blow. But, there is a label on the valve mentioning about vacuum pressure. Not to exceed 8 psi.

Here is my theory. The rear tank gas cap is not vented to outside. The new cap for the front tank appears to be vented. Was there pressure in the rear tank that in some way overcame the solenoid valve and allowed fuel to siphon over to the front tank? The rear tank cap is now vented as well, but I have not test drove because I noticed fuel had started spitting from the fuel pump sight hole and I am changing the fuel pump. Luckily I already had a new spare pump.

Anyone ever encountered this fuel siphon with dual tanks?
yep that's exactly what happened. had it happen on a recent maiden voyage/road trip on 40 with dual tanks. rear wasn't vented and it would build so much pressure it would fill the front tank. I had the auxillary (rear) tank plumped so it would fill the primary tank (front) when I switched the auxillary pump on. after venting both tanks problem solved.
Thanks for the reply. It was very surprising to open the cap and see fuel right up to the top. I expect my test drive will go much better.
Actually, if you had tanks plumbed smog legally, that's exactly what would happen. You'd run the front tank to empty, then switch to aux. tank. At that point rear gas would be suctioned into front tank, and partially replenishing front tank as engine also burnt gas from front tank. Problem with this scenario is that you never really know how much gas you have. Best method is to plumb non smog legally, both on their own sender, both with vented caps, run one tank or the other. Use a DPDT switch that switches tanks AND senders at the same time- - - easy deal.

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