My dad gave me his company work truck.. it's an '87 F150 I6 4 speed with the dual gas tank setup.

The gas gauge is funky. Showing half full on one tank that is empy and showing empty on the other tank with gas in it.

In getting the muffler and tailpipe fixed to pass inspection, I noticed gas flowing out the front tank. At first I thought it was overflow since it was very full--I thought I ran out of gas a week ago but discovered the tank was full after trying to dump another couple gallons in there. I assumed the intank low pressure fuel pump was bad, but not I'm not sure.

Any help is appreciated..

 

This is a known problem with the change-over valve assembly or check valves, depending on what year. Different systems were used for different years. Believe there were TSB's about this. I have a single big tank, and don't remember enough about the 2 tank problem solution. But your observation fits the problem exactly. Hopefully somebody here who has dual tanks can help. If not, more Internet research...

 

I asked a buddy who is very knowledgeable with the Ford fuel systems. He immediately said the fuel resorvoir was the culprit. It is bolted along the frame rail right before the high pressure fuel pump and has 6 fuel connections to it. It's causing the gas from the rear tank to be pumped into the front tank and ultimtely overflow.

I got a price of $60 from a Ford dealer locally. (Sunset Ford in St. Louis, MO)