I just got my 89 f-150 4x4 on the road. It runs well, but sometimes when it is running, it will stumble really really hard and sometimes die. If it stays running and you try to give it gas, nothing happens. I installed a new pump assembly and it seemed to do fine until now. Could there be crud in the lines or a plugged filter(that would affect both tanks though). The truck was sitting since 2002.

 

Which pump did you replace? On your truck there's one in the tank and one on the driverside frame.

If the problem is intermittent, the best way to diagnose would be to attach a fuel pressure gauge to the fuel rail that has a long enough hose and tape it to your windshield and verify the pressure while driving.

 

Both, anf the relay. If I switch to the rear tank, it works fine. The thing that comes to mind is that I at one time had to remove the first new pump/sending unit that went in the tank, I set another one in there, but never locked it in... then it snowed. Could some water in the fuel be doing it? The thing has only been driven 54 miles since I got it.

 

So you're saying you have two tanks. Both have new in tank pumps but the truck only runs when you switch to the rear tank?

If it runs fine on the rear tank, it can't be the fuel filter.

In the side gas tank you have a new pump but didn't install the locking ring and when you switch to it, the engine stalls?

I'd install a fuel pressure guage on the fuel rail and switch to the side tank and try and start the engine. It may run for a minute using up the fuel in the selector reservoir but if the side tank internal pump is bad the fuel pressure should drop.

 

Here is what I did, as follows: rear tank replaced with a take-off from another truck( works, verified fuel flow before connecting fuel lines(turn key on and watch gas come out), New pump in front tank, which was later scavenged for another truck(I set the old pump from my other truck in the tank to keep dust out, but it was not locked in with the ring, so some water probably got into the fuel during a couple snow storms.), I have since installed a new front tank pump... again, and tested it with the key before ever connecting the hoses. I also installed a take off frame pump. The truck was fine and did not stall while driving around my yard for a month. It is now on the road as a daily driver. It will run fine for about 4 miles or so while switched to the front tank, then, sometimes it will stall out, sometimes it will not. To fix this, I switch to the rear tank and it will run fine, a minute later, I can go back to the front tank again and it runs like a champ. I would figure either water in gas, or a clogged fuel line from the front tank.

 

Yes, im betting water in the tank, too bad these trucks dont run good on water. STP makes an additive called all season water remover, might be worth a shot.