I have a 94 F-150 4x4 with dual tanks and the front tank will build up pressure. And it will start to leak out of cap only untill I relieve pressure. What is the cause?

 

It's a known problem for that era of truck to transfer fuel from one tank to another, but opening the cap won't transfer it back, so IDK why that solves your problem (even temporarily). The common fault is in the check valve in the top of the FDM, and the only fix is to replace the whole FDM (fuel pump/level sender assy) with a Ford part (but you can get it from RockAuto, 1stFordParts, etc. cheaper than the dealer).

For more info, click my signature link & look in the EFI Fuel Pump album. It shows a Bronco, but yours is almost the same.

But if popping the cap really fixes yours, it can't be that check valve. I can't imagine anything that would pull fuel up into the filler neck when the cap is on, other than MAYBE a bad cap vent. You could try replacing the cap, but I've never heard of one doing that.

 

It might be the vent in the OTHER tank (the one fuel is leaving). If it's clogged or inoperable, the pressure may force the fuel into the other tank.

Just a guess, and would be easy to fix.

Good luck,

Lee

 

That's not possible. The check valves close when the pumps aren't running, so there's no way a gas tank could withstand 1/4 the pressure it takes to open either valve; much less, to open BOTH check valves.

 

I searched for the fuel level assembly and cannot find it do you happen to have a link to it?

 

What do you mean? The location of the level sender & how to change it, or a source to buy a new one?

 

All the above where is it how to change it and where to find one. I think that I am loosing fuel from rear tank into front Last time I switched over I believe I had approx. 1/4 tank left I switched over back to front today I found I had just under 1/2 is this possible?

 

Click my signature link & look in the EFI Fuel Pump album. Buy a new Ford/Motorcraft FDM from the dealer, or some other supplier like RockAuto or 1stFordParts or a vendor here.

The one that needs to be replaced is in the tank that's RECEIVING fuel - not the one that's losing fuel.

 

I have searched for them at RockAuto and 1stFordParts and It wont pull one up it leads me to the fuel pump. Any added help with this? Sorry to keep bothering you

 

They're calling it a fuel pump on 1stFordParts because most people don't know what an FDM is. I think for $255, you're getting the whole thing, but call your local dealer & see what their price is. It'll probably be ~$350.

The aftermarket ones at RockAuto are FDMs without the mounting plate or steel tubes, but they'll fix your truck. The ones I saw are under 1994 FORD F-150 PICKUP 5.0L 302cid V8 MFI (N) : Fuel/Air : Fuel Pump and were $90-250.

 

Thanks