Adam Rouillier

I have a 1996 F-150 with dual tanks. Both tanks work proper including the fuel gauge. The problem I am having is that when the truck is parked and off (or at least it seams that way), the fuel in the rear tank moves to the front tank until it is full or the rear tank is empty. The front tank does not overflow. Has anyone else had this happen and know what the issue is?

Chris_1

iTrader: (1)

Wouldn't be likely to happen when the truck is off; more likely to be happening when the rear pump is running. Called cross-filling which happens when the check valve or shuttle valve in the pump not being used (in your case the front pump) is faulty allowing fuel being pumped from the rear tank to enter the front tank.
It's possible I suppose that it continues to siphon after you shut the truck down if the rear tank was higher; but more likely to happen when the rear pump is running.
If the front tank isn't overfilling - that you can see- it may be going out your vent tube at the top and being pushed up to your charcoal canister (the black plastic box below your battery).
Generally the only fix is to replace the whole pump unit in the tank that is being filled.

The other thing it would be doing besides displacing gas between tanks is reducing the amount of fuel pressure at the fuel rail which results in a drop in engine performance.

Chris_1

iTrader: (1)

If you're not getting any cross-filling to the rear tank when the front pump is in use, the best thing is to just use the front tank for now until you can get it fixed.
Sounds backwards but that's the way it works.

Adam Rouillier

Thank you for the reply. I u understand what you are talking about but there is nothing coming out the charcoal canister, at least not a vusuabke liquid. Secondly I run off the front tank until it is nearly empty and then find the rear tank at the same level. Does this make any since to you? The statement about the fuel rail makes since as I just rebuilt the motor and it was running lean and hot. The valves were white indicating high heat and a truck running lean.

Chris_1

iTrader: (1)

Maybe what you should do is put a 1/2 tank in each one then put a fuel pressure gauge on it to see what each pump is actually doing as far as pressure.
I know a fuel pressure gauge costs 40 or 50 $$ but it's a good investment if you own a fuel injected vehicle.
A gauge is a really useful tool to have - you will use it again and again. There are loan a tool programs but you will probably spend more in gas picking it up and taking it back every time you want to use one than if you just bought your own.