Hey folks, just joined tonight and any help will be much appreciated. I have an 87 F-150 with dual tanks. The rear works fine, but when I switch to the front it pulls gas from the rear and overfills the front to the point of blowing gas out of the filler cap. Any suggestions?

 

Welcome to FTE! The problem you describe is referred to as "cross-flow" and it is a VERY common issue. The Dual Function Reservoir is to blame. It's mounted on the driver side frame rail. The most likely to fix it is to replace it.

Some info on the single and dual function reservoir from Steve83 here: 1983 Ford Bronco '84-89 Fuel Reservoirs pictures, videos, and sounds | SuperMotors.net

Your truck uses two low pressure in-tank pumps, a dual function reservoir and a single high pressure pump mounted on the frame rail. Again the most likely cause is this reservoir. Steve's album should have the part number for it. You can usually find them listed on Amazon for ~$200.

If you use the search function on this forum you will see hundreds of posts from others with the same problem. Just keep in mind the 1989 and older trucks use this dual function reservoir/3 pump system. The 1990 and later trucks use 2 high pressure in-tank pumps without a reservoir, but they also suffer from the issue but different root causes.

 

thanks. I will check into it.

 

My blue truck has dual tanks and the previous owner removed the dual reservoir from the frame rail and simply straight-piped one tank into the lines going to the high pressure pump. The truck was driven like this for a couple of years.

When I got the truck I thought it strange to have the low pressure pump feeding the high pressure pump without the reservoir. I had fuel delivery issues and I thought this might be the problem. So I bought a SINGLE reservoir from LMC Truck. It was NOS, original Motorcraft part. It was around $90. So now my dual tank truck uses only the front tank to go to the single reservoir, then on to the high pressure pump. You can get a dual reservoir from LMC truck, it just costs twice as much.

And that link to Steve83 is a wonderful explanation for how the system works. It really made a difference to me when I was troubleshooting mine. Good luck!