I bought a 96 F150 with a 4.9 for not very much money, I had a 95 F150 - 4.9 that was the best truck I had ever owned. The truck would start and run for a couple of seconds and die.. Having brand new tires, brakes and complete new front end parts and rebuild and many other new parts, I deducted that the truck had a fuel problem even though they had replaced the fuel pump and filter (I saw the filter and it was really nasty but they didn't clean or replace the tank). I figured if it was going to be a money pit then they had already filled most of the pit (what I get for figuring). I bought a new fuel pump and fuel tank and had a shade tree mechanic put them on. It started and ran but it had fluctuating idle.. We put the scanner on it and it would show power but not transmitting codes. The kid I bought the truck from said his grandfather had put a new ECM on the truck before he died with covid last winter. I took the truck to a real mechanic, who I trust. He called me and said he could continue to replace parts hoping to find a fix but there is a deeper problem that required an electrician/computer expert. We have an auto electric and computer guy in our town but it will take about 3 weeks to get in. I really would like to use the truck and don't mind spending some money to get it right but I also don't want to be a fool. I know there is no way anyone can diagnose the problem, I guess I'm just looking for advice or input. Bail or ride it out. Right now I'm at about $1500.

 

I would try another ECM. Maybe the one on the truck now can be reflashed if you can find someone to do it. I had similar issues with a 89 Corvette last year, after replacing the ECM, it still had rough idle , ran fine under throttle. I was able to find a thread on the Corvette forum that was similar and that guy's fix was to use the original MAS that the factory used ,that being a Bosch. I found a NOS one, installed it, the car ran perfect and the service engine light went out. Good luck !!