Ok so here it goes. I lost being able to use the rear fuel tank in spring. Took it to my "Regular Mechanic" who said it was BOTH the inline HP pump and rear tank pump. I begrudgingly paid the man and got the truck back. It died in the parking lot on the rear tank. Gave it back to him and asked him to check the fuel selector valve. His response was to the effect of *Those never go bad* (absolute BS from what I've read).

Anyways After fighting with him over it I took the truck back and started working on it myself. I have since went and paid the 300$ for a new non motorized 6 port diaphragm type valve, yet another fuel pump LP pump motor *just in case the one he put in was defective*. I've even tested the pump with the fuel line off and it seems to pump fine. I replaced the dash switch and checked the voltage coming out of the new switch just in case.

So with the knowledge I put in a brand new valve new pumps all around, new switch. What do I have left? When I turn the truck to the rear tank I get maybe 2 minutes on the reservoir before it stalls out.

Do I need to start changing fuel lines at this point? Or should I hunt for a used valve?

1987 F150 Long bed 4.9L I-6 Auto Trans

You guys are my last hope here.

 

the problem is only with the rear tank?

 

Obviously you have changed the fuel filter ?

Then, I would check the pressure at the fuel rail in the bay to see how it behaves.

Otherwise check or replace your fuel tank filler caps as it may be a pressure problem.

There are also vent valves on each tank that could be bad or those lines could be impaired.

 

Check my 88' thread for some pics of a fresh rebuilt system, for clarity.
https://www.ford-trucks.com/forums/1...th-saving.html

 

The problem is ONLY with the rear tank. I have replaced the fuel filter and the sock in the tank.

In regards to pressures I was getting about 45-50lbs fuel pressure on the front tank.

It stays for about a minute or two on the rear but after the reservoir can runs out of fuel the fuel pressure drops to zero.

Fuel tank caps behave as they should. The problem happens with the cap on or off.

 

Possible that you have a leaking seal(s) at one or more of your fuel line connections.
There are two (2) orings at each pressurized fuel line connection. they are each a slightly different size oring.
On my truck which had sat for 16 years, they appeared good and I only replaced two o-ring connections before I had the thought that it was unnecessary (we will see) but still wonder how well they will seal.
Maybe try to get in position to look for visable leaks, though they might not even show on the outside. Sourced my o-rings from McMaster Carr and they are , of course, fuel safe. Probably have the numbers somewhere if you need them.
Luck, you can do it.

 

Is the rear pump getting power? I would suspect a switch in the dash or bad wire/ground. Verify with VOM. If not that, a collapsed or clogged line. Try blowing out both ways with air.

 

Rear pump is getting power, The switch was replaced. I'm leaning clogged line at this point.

 

Blow em out both ways... return lines too and could be a collapsed line too.

 

The question that needs to be answered is how long does the rear in-tank pump run because it would seem it is quitting pretty quickly.. only to run fine again after "cooling down" for a short period.

OR

Is the rear tank not venting properly, the pump won't continue to push fuel if it's operating in a sealed or almost sealed container.

 

More than likely a bad connector plug at the top of the rear tank.
Put a new pig tail plug on the wiring to the rear tank.

Edit:
That was my first thought.
But after switching the truck should run OK more or less with the rear selected pump not running.
So the rear pump does run and switch the selector valve or it would not die.
If the valve did not switch it should still be running off the front tank with its pump not running.

So you have a problem with the selector valve.
The valve more than likely has been setting in stock for a lot of years and now has a problem with stuck "O" rings.
You are not the first poster on here to have problems with a new selector valve.

OR
You could have junk inside the rear tank clogging the sock filter after it runs for a few minutes.