So I picked up this 84 F250 with the 6.9 idi, the truck ran fine for a couple days but now when I drive it around for more than a half of an hour it starts lugging and surging.
She fires right up every morning no issues and doesn't start to run rough untill I have been driving for a bit.
After driving for a while the sputtering will get so bad that the truck will die and I have to go and bleed a bunch of air out of the Schrader valve.
So of course im having some kind of air intrusion problem but I also think it might be the lift pump.
Im unsure of what exactly is happening or why the truck runs fine for a while with such bad air intrusion.
I can do small trips around town but heading any distance is a bad idea because once it dies, after I get it running again, I will be dying every couple of miles and have to repeat the same process all while the truck is running like complete garbage.
For now Im going to run a outboard tank to the lift pump and see how that dose but this is just so weird.

 

Another forum member had this same problem... I believe he eventually diagnosed it as rust induced pin holes in his steel fuel line hidden by his midship fuel tank.
Just for reference and trouble shooting purposes:
A) If you have hard starts/long cranks after the truck has been sitting for a few hours/days, than you have air leaks in the low pressure side of your fuel system. Check your return caps, your rubber return hoses, your fuel line olives, or your fuel-heater o-ring on your filter head for any wetness.
B) If your truck stalls while driving down the road, you have a air leak on the suction side of the fuel system, somewhere between the fuel pick-up inside the tank and the lift pump.
C) If you shut down your truck and it won't restart when the engine is hot, but it will start when it's cold, then your high pressure injection pump is getting weak.

 

SOLVED!
Aftermarket fuel filter and water separator where on the truck and it had been clogged. I bypassed it because I was on the side of the road and will add a large inline filter in the future.

 

How I "solved" the issue until I could get to a garage.

 

glad you got it figured our, and it was a cheap fix.

 

Can you get a photo of the aftermarket filter and water separator. Was it frame-rail mounted?

 

UPDATE
The mechanical lift pump was also having issues with pulling through the aftermarket water separator (it was causing a vacuum in the fuel water separator) and I just ended up using an electric fuel pump that is used on ford B series busses and placed it close to the fuel tank and used a big block chevy mechanical fuel pump block off plate where the lift pump was....

 

What type/brand Fuel/Water filter separator was on the truck?

Iam using the Racor 500FG and stock lift pump and having no problems.
Good you finally got it worked out.

Charlie

 

Just a standard looking filter housing that takes a NAPA GOLD primary 3123