1990 W250, 255k miles, bone stock, no mods.

First a little history:

This engine has always started on the very first crank. The other day I let a friend borrow the truck to move some stuff. She drove it to her house, loaded it up with her stuff, and then I get a phone call from her saying it wont start. I thought she must have run it out of fuel. She said it was at 1/4 tank, but her driveway is a hill, so I still thought she might have sucked in a little air.

About 3 hours later I went there with 5 gallons of fuel and a can of ether. I put the fuel in, opened a few injector lines, gave it a shot of ether, and it fired right up. I shut it off, and restarted a few times, and it seemed back to normal. Every time starting on the first crank.

She then went about with her moving, with about 10 miles of driving, and then I get a call at 11pm, she is at a gas station and it wont start. I drive there and do the same routine I did earlier in the day (open injectors, pump primer, ether, etc.), but now it wont restart. After many tries nothing was happening. I remembered I had a spare fuel filter behind the seat, so I filled it with fuel and put it in. Then the engine started the very next try.

I was feeling confident that I had now solved the problem (plugged filter). But over the last few days it started normally, but I didnt drive it.
Today it started normally and I drove about 1 mile to a neighbors house, shut it off, and now it wont restart.

Ive searched the forums and tried most everything suggested.

There is 12 volts at the fuel solenoid.
When I open the injectors and crank the engine, I get little spurts of fuel.
Ive pumped the primer until fuel comes out the bleed screw.
Ive checked all the lines with a flashlight to look for cracks or signs of leakage.
Ive greased the fuel heater connector.
Ive unscrewed and reinstalled the filter, It is always full of fuel. This filter has about 5 miles driven on it, so I really doubt it could be plugged.
when I pump the primer lever, I feel resistance for about five pumps, then it starts to feel like it is no longer pumping.
I give it a little ether to help it get started but its just not getting fuel.

Its acting like it is sucking air/losing prime.

Im starting to suspect the lift pump, but I want to be sure before I start throwing parts at it. Any ideas?

 

I have seen the solenoid valve on injection pump go bad. The only way to test is remove valve and spring then reinstall solenoid. Vehicle will start if solenoid is bad. To shut down engine use manual lever on side of pump and turn off ignition key.