i installed a aux fuel tank in my bed under the tool box. im using a holly electric pump to pump the fuel into my fuel neck hose which i have tee'd . it works great but i have noticed that ive driven 75 miles and the main tank is still full but the aux tank is going down. with the electric pump off, the main tank seems to be siphoning the fuel out of my aux tank. not sure how to fix this. i thought about drilling a small hole in my main cap to allow air to enter the main tank which would get rid of the siphoning but wasnt sure if this would set a code or light. i know on my alero if the cap isnt install it with set a dash light. i cant seem to find a electric fuel on off switch that would stop the flow when pump is off. please any suggestions would be great for im at a lost.

anyone?

Drilling a hole in the main cap is not a good idea. It could allow fuel leakage. The situation you describe would suit me fine... Run off the aux fuel first, then the main tank becomes a reserve. Once the fuel gauge starts to drop, you know the aux is empty and can plan your next fuel stop accordingly. Why do anything?

I have a setup like yours but I have a soleniod valve which opens when I turn on the pump other times its closed. This will stop you problem.

My setup is similiar to yours. I do have a loop in the aux fuel line, and I also put a tap in the filler neck, so the input for the aux tank is high up on the factory filler neck. Mine does not siphon.

is it overflowing if not why worry about it let it fill up youer tank with the gauge in it