1990 F350 7.3 IDI I bought as project truck needing work. I replace one head due to a crack. I put everything together, replaced all leaking fuel o-rings, etc on top of engine. I don’t see any current leaks. I’ve been driving it around for about 150 miles and down the highway it feels like it surges just a little. Throttle response seems really quick, if I barely push on the gas the engine revs quick. This makes it a little difficult to drive slow like thru a neighborhood. Previous owner retrofitted a 4 speed. It somewhat feels like an old gas engine that has a slight vacuum leak where the engine feels like it runs a little erratic and the RPMs go down kinds slow. I’m not sure if the intake could be leaking air in, or maybe the injector pump timing needs to be adjusted. I’m also thinking I still need to burn out some crappy fuel left in the tank. I’ve gone thru about 3 tanks fills and I’m adding diesel fuel treatment for water. Any ideas or where to start? Thanks

 

That's air in the IP. Press the Schrader valve on the fuel filter header while the engine is running and it will most likely blow a bunch of air out.
The problem could just be that there was air in it when you put it all back together and it just needs to get rid of the air .
OR there is a small hole somewhere between the tank and lift pump, and you are constantly getting a little air with fuel.

I'd just purge the air and run it, see what happens. This actually just happened to me not long ago. Got air in before I could switch tanks and it stayed locked in and drove just like you are describing. I only drove it maybe 5 miles not 150 but same feeling.

 

Ok, so far I replace the rubber fuel hose from the frame line to the fuel pump, from pump to filter head. I bled the Schrader valve. I have a clear line from the filter head to the return lines and it still has air in it while truck is running. Shouldn't this line be solid fuel with no air? I'm not sure of a good way to check the lines for air leaks to the tank and to the selector valve.

Thanks

 

Yes that line should be (mostly) full of fuel with no air. A few little bubbles might be in there but that's kinda the point of that return.

Does it still run poorly?

 

Might want to replace the lift pump($25), just in case. It might well have a leak to the inside, and is sucking air with the fuel.

Other than that... you can A, run a line from a jerry can to the fuel pump to check the whole system, or, say, cut the line between the rear tank and selector switch, add a piece of rubber hose and pull from a jerry can to eliminate the tank itself... (patch it back with a short piece of rubber hose)...

You can also drop the tank(or pull the bed, might be easier), and get access to the sending unit, start going over every inch of line.

Chances are, it's either fuel pump, pickup, fuel selector valve, or a loose connection/O-ring.

 

I ran a rubber fuel line from the pump inlet to a jerry can. I ran the truck and bled thru the Schrader valve really well and the clear return line is still is full of air. I see the fuel squirting in from the filter head but its like it drains off so quick it doesn't fill up. I bought a new lift pump for it about 6 months ago, but maybe it is bad. I can't remember if I bought at Rockauto or a local part store. How common is it for these pumps to start sucking in air?

I think I'll buy a cheap clear inline fuel filter and put it in line right after the lift pump and see if its showing air.

 

If the engine runs well and the Schrader is pissing fuel and not air, then you're good. The clear line will never fill up since its just open to drain back to the tank.