87 F150 dual-fuel tank diagram?
 

Im having all kinds of issues with my dual fuel tanks....Ive searched and read about the 'selector valve' recall, but I dont think it applies to 87s, from what IVe read....

Can some one briefly explain how the fuel is routed in this system? Does the rear tank just fill up the front tank in-series, or does it go separately to the engine? When I flip the dash switch to "R", it continues to draw fuel from the front tank only, and never draws from the rear tank....leading me to bel that the switch just shows the level-indicator in each tank and doesnt "switch" the selector valve from one to another....is that right?

Can someone point me to an overall diagram of the two-tank system, ie, path of the fuel from each tank, route of fuel thru the selector valve, what fuel pumps pump what, their location, etc.....thanks!!!

I don't know of a diagram, but I can try to explain the system to you.

There is a low pressure pump in each tank. The supply lines run from the tanks to the selector valve, which is operated by pressure from the in-tank pumps - the valve controls the supply side as well as the return, so that un-used fuel from the fuel rail is returned to the tank that is supplying the engine. From there, the supply line out of the selector valve goes to the high pressure pump on the frame rail, then through the filter, then to the fuel rail on the engine, then past the regulator back to the tank, via the selector valve.

Electrically, the switch in the dash switches both the fuel level indicator and the supply pump. 12 V is supplied from the fuel pressure regulator, then to the inertia switch, then from there it splits - one line runs to the high pressure pump, and the other runs to the dash switch, where it is then switched between the front tank pump or the rear tank pump.

If it only draws off of one tank, then I would suspect a failed in-tank fuel pump in the tank it does not draw from (no pressure to switch the selector valve, and the high pressure pump is sucking fuel from the tank with the working pump). Have someone put their hand on the fuel tank and feel/listen for the in tank pumps to come on, right when you first turn the key to the run position (don't start the truck). Try this on both tanks and see if you can tell if one of the pumps is not working.

Sounds like the change over valve has failed or the rear pump went out and someone just jumpered the switch and bypassed the valve to use the front tank only (cheap or lazy). Could also just be a failed switch, rare but I've seen it.

Fuel selector valve bad??
 

When was the last time the rear tank was used? If it hasn't been to long, i guess you could switch the top two fuel lines with the two bottom fuel lines on the fuel selector valve to see if the front tank works?? Top two lines or for the front tank.But if the fuel selector valve is bad it may not allow fuel to your main external fuel pump, and you could burn it out. On my 87 f150 4.9 efi my fuel selector valve was bad and would not allow fuel from my front tank into the engine. All the fuel from my rear tank would get pumped to the front tank in about an hour, then when i switched to the front tank it stalled. Have you noticed fuel switching tanks, or overflowing or build up of pressure when you open the fuel caps? Recall for for 89's which had the same part, i guess because of the age of the truck the part is old.The valve cost about 65.00 at fords, i could not find a aftermarket mechanical ( no wire to it) valve. In your cause it may not be the problem. How many mpg do you get on your front tank, 19 gallon front tank at about 13 mpg is about 250?? If you get about that, then the selector valve is maybe ok?? When did your problem start? Sounds like your truck runs ok besides that? Maybe you could put some mystery oil in the front tank to keep the fuel from going bad??

Ive had the truck about 3 years, and I cant *confirm* it has ever pumped fuel from the rear tank. For a loooong time it would just stumble and die if you put it on the rear tank, so I just used the front tank exclusively. From time to time I would flip to the rear tank and it would run for a little while, then stumble-die, so I flipped back to the front tank and all was well. As I only drove around town, it wasnt an issue not having two tanks.

I went on a 600-mile trip last week and wanted to use both tanks. I filled both and started on the rear tank. It ran fine, but after a long time, I noticed that the gauge wasnt dropping at all, stayed at like 3/4 tank. I was puzzled but decided to go on and see how long this would continue. After about 250 miles, the engine started to stumble, so I flipped to the front tank AND THE GAS GAUGE WENT TO ZERO. That's when I found out it had been drawing from the front tank all the time, even though the switch was on the rear tank. The rear tank was still full when I stopped at a station. :-huh

As I am going to start using this truck for more long trips, two tanks are important now...so Im trying to figure out how best to attack this problem. I appreciate the fine responses so far! Thanks! All ideas are gratefully accepted....

So when you add fuel to the front tank will it run? Is there any chance that the switch on the dash has been worked on, rewired? You can reach up on the front fuel tank and lift up the plug and check the voltage. Did you hear the fuel pump in the front tank come on, with the key to on? You may not want to use the fuel gauge, mine gives me false reading all the time. We can figure this out.
Some people have luck with removing the fuel selector valve and opening the bottom to see if there are any loose parts, open the top and clean it out and reinstall, may work.
1987 F150 4.9 EFI
Mike

My rear tank was doing the same thing.
The problem was the wiring on mine.
Cindy 88 F150