Just purchased 86 XLT . Last owner used just the rear tank. The issue I have is a build up of pressure in the front tank. Any trip I smell gas when I get out of the cab, If I remove the gas cap ,gas pops out and run down the side. I have remove a 1/2 gallon from the full tank. Also if I flip the switch from rear to front tank the gas gauge drops to zero. Any suggestion ?
Thanks

 

A little info and history if you would please....What engine and trans, and are the original emissions systems still intact ? By the sounds of it, there at least two, maybe three different things going on with it.

 

The truck has a 302 with automatic. According to the recent safety inspection it had all original emissions equipment. I manage to siphon out a gallon of gas. The last owner said he never used the front tank make me wonder why ? Thanks' for any suggestion or ideals

 

The truck has a 302 with automatic trans, According to the last recent safety inspection it had all of the emissions equipment. According to the last owner he only used the rear gas tank which seem odd. Thanks for any suggestion or ideal where to look for the problem.

 

The gas tanks will have a vent tube (vacuum line) that goes up to the engine compartment, through a charcoal filter, and eventually to the carburetor. This is to burn off excess gas fumes since gasoline evaporates at room temperature. They added this as part of the emissions equipment starting in the early 80's. If you are building up pressure in the gas tank, then this vent line is likely blocked.

You can trace the line by starting at the engine compartment. Find the line connected to a charcoal canister and trace it back to the gas tanks alongside the passenger frame rail. Make sure that they haven't plugged the line. If it's plugged or the charcoal canisters have been removed/etc, you can probably just remove the line and let it vent to atmosphere if you don't have emissions testing in your area.

If it's not plugged and looks connected properly, you might check/replace the evap purge valve (looks like a mini starship Enterprise) between the charcoal canister and the carburetor. The part number will be on it.

If that's not it either, follow the vacuum line from the purge valve up to the carburetor. If it's not connected, connect it. There are other vacuum control valves it might go through also (these make sure it only sucks fumes off once the engine is warmed up or at certain RPMs). They need to be working too.


Evap/charcoal canister


Sample purge valve (could be red also)

 

Braggs did explain one of the possible issues so I'll move on to the others that I was considering.
The second consideration is the fuel vent tube at the filler neck itself. They have been known to dislodge, break, rust through, etc. (the reason I asked about the pump shutting off prematurely). Concerning the gauge dropping to "E" when the tank switch is engaged to the forward (midship) tank, this could be due to the level sender simply being inoperative from little or no use. The fuel tank selector valve or it's electrical connection could also be a concern.
Does the truck run when the front tank is selected or does it quit ?
Here's a pic from the FoMoCo parts catalog showing tanks and filler necks :





I would also suggest that you have a look at the site at the link below. I would have posted the illustrations from it here but it is a lengthy page.

http://www.garysgaragemahal.com/1985-on-f-series.html

This is a great site created by a true F-Series 80-86 enthusiast.

 

If you have the 302 EFi engine then the tank switch valve may be faulty.
The tank switching valve Ford calls a Dual Function Reservoir is switched by fuel pressure from the in-tank low pressure fuel pump of the selected tank.

In order for the DFR to work properly both in-tank fuel pumps must be working.

If the valve is stuck or broken then fuel can be pumped from one tank to the other and/or the excess fuel returning from the engine EFI can be returned to the wrong tank.

I got rid of the DFR system and put in a electric motorized select valve.
I have some info on my web site but note that my photo web host deleted all the photos and I haven't got around to fixing them.