New to me 1997 diesel can it be saved?

Nice extra cab caught my eye. I should have passed but current truck is rotting quickly now. This new one the frame and body rust free except for some bubbles above the windshield, 158k miles new exhaust and tires. Was smoking and knocking really bad but a recently replaced long block. I rolled the dice and got it very cheap, just over $2k. Guy said it never really ran right since the new long block was put in, I am hoping he just had a really bad mechanic. So far I found that the oil pressure sender wasn’t connected, an injector return line wasn’t connected and the pcm they reinstalled under the intake was failed. Even though there was a relocated on under the bumper that just needed a new cable. Fixed these and smoking is mostly gone knocking is almost gone but still there. I replaced the fuel filter and I suspect the fuel is 1 year old anyways. Put some additives in. Fuel at 1/2 tank. Going to replace the oil pressure sender because it reads good but after idling for awhile it drops to like 7 or less something is going on with it. What I do need some help with is the blue plug going into the PCM. Looks like the PO frayed 2 wires and tried to fix. They may be working but I have no idea what they are for. It’s a brown/white stripe and a black white stripe on the blue plug.

The ecm box circuits look good, I get a pump timing slightly off code after ideling for a half hour or more. Should I just run this fuel out and it my clear out?