I'm going crazy!!!! A few weeks ago I was running just fine on WVO and when I went to switch back to diesel the truck died within a few seconds. I've been over everything I can think of with the tools I have (limited budget) and I feel like it should start, but no fire! Please Help!!! Here are the details.

Potentially pertinent history:
I ran out of gas a month or so ago. Both fuel gauges don't work. I know running out is a no no, but I messed up. In the process of getting it running again I killed the starter. Replaced that and it fired right up. New starter seemed to crank faster than the old one. Got 2 new batteries (a few months old). Lately I had been noticing the tach doing funny things. It was maxed out when driving down the road at 30mph but clearly the engine was running normal rpms. After warming up a bit it seemed to smooth out, but would still jump around occasionally. Now the tach doesn't move when cranking. Also, the last time it ran well I had a hard time getting it started (5 or 6 cranks, then good to go once running). The Veg is heated in 3 places: tank, veg filter, and veg. therm in line just before it hits the IP.

What I've tried:
This truck is a hobby for me and I work, "off the grid," every other week so it only gets driven a few times a month. At first I thought there may have still been air in the lines from the initial run out. I cranked and cranked while cracking each line (except for #1). I also tried squirting liquid silicone into air intake to get it to run a little and save some cranking. It would occasionally fire for a few seconds, running real rough, and then die. When it did fire there was no response when pushing pedal to the floor (sounds like a definite fuel problem right?). I've changed the fuel filter (I filled it with diesel) and then bled the lines again. I tried running a hose from IP intake directly to a can of diesel... No start. I disconnected the fuel line into the fuel filter and cranked for 10 seconds. Plenty of fuel. Question: I don't have a pressure gauge, but I assume if I get at least 1/3 pt the lift pump is good. Do I need to test pressure there too? I reconnected that line and did the same test with the line coming out of the fuel filter. Plenty of fuel. Just to cover all the bases I tested all the glow plugs. I did a resistance test with them still in the head and got 0.7 ohms or less on 6 of them. The other two I replaced. I tested voltage at all glow plugs and got about 9.6 at all 8. Glow stays on for about 10 seconds at on first glow. When I had each line cracked I appeared to be putting out plenty of fuel, but I don't have much of a reference. It's more than a few drops though. Does anybody have a good video of what it should look like? I have many of the fuel lines replaced with clear. I have no bubbles that I can see going into the IP. I have cracked and bled the lines quite a few times at this point. I do have a few small bubbles in the return line after each crank. A friend told me this was ok. I'm assuming it comes from when the injector pops open and closed. Question: When retightening the steel injector line on #8 there seems to be a 1/16" gap between the nut and the plastic return line cap. This looks off, but I can't seem to get it any tighter/ don't want to over tighten. I've been seating the nut and then about a 1/4 turn. I tried a bit of wd-40 the threads, worked it back and forth, more wd-40, a bit of brushing, back and forth, and so on. Still stops at same place. Is this normal?

What I'm thinking

It sounds fishy that it died when I switched fuels. I am assuming that the 1/3 pt per 10 second crank tells me I'm getting enough fuel going into the IP. Is this a safe assumption? As I was writing this I just thought about the o-rings in the cap that everybody talks about. I would do the return kit, but with my limited funds I don't want to go buying stuff that I MIGHT not need. If the O-rings are worn could this cause the cap to sit lower, creating the 1/16" gap? Would this cause a not start? I also watched a video where a guy cleaned the check valve on the return line to the IP. Gonna go check the O-rings on the #8 cap and maybe clean the IP return line check valve while hopefully somebody responds!!!! Thanks for reading all this. I tried to be detailed in my first post to avoid too much need for back and forth. I was told when I bought this truck that WVO would eventually kill the IP. I'm hoping that's not the case. How can I test the IP?