TBI 454 Help

[F85Cutlass]

Hello, my name's Ryan, new user with my first square body. 89' V3500 TBI 454, TH400. 66k on the odometer.

Been on quite the gremlin hunt going on a month. When I first got the truck, it had been sitting for 7-8 years. Gave it a tune up, and it ran beautifully, which was my go ahead to begin going through the brakes and some other projects on the truck. Of coarse, after a month or so of runnign well, and being yard driven, I register it, and then it starts acting up.

While Idling, I am topping off the trans fluid because I had the lines off, and it quite instantly starts to idle in a rythmic surging manner. Thinking that it couldnt be a coincidence that this began, I went about testing vaccum to the transmission pressure transducer. With good vaccum there, I proceeded to replace the transducer and do a transmission fluid/ filter service. The issue remained.

I did eventually get it to throw a len mixture code, which at least gave me something to go off of.

I proceeded to to check and replace vaccum lines. The only difference came from caping off the MAP sensor, which to my understanding puts it into a kind of rich mixture default. With the map capped off it ran well enough to chug in and out of the garage, but that was about it, no guts whatsoever.

My understanding is that this rich default can mask poor fuel pressure, so I turned my attention to the fuel system.

As many of you will know, GM did not give us a schrader valve to check pressure. To make a long story short, I got an adapter off of summit, which was garbage, leaked from the swivel, and did not give me a read out, so I just went about a fuel pump replacment, bc I had a hunch. Turns out the hose between the pump and sending unit was split, which I belive was resulting in just enough fuel to run, but not enough to run well.

So, I replace the pump, hose, strainer, ect. Now truck will not start, but now it appears to be dumping excess fuel. Will almost catch if I hold throttle wide open. Not sure if theres some kind of ecm relearn process, but battery was disconnected during the fuel pump replacement, so I would think it would start from a default fuel trim at this point anyway.

I did end up replacing the throttle body with a remanufactured unit before I got to the fuel pump because while attempting to test the IAC valve and TPS, the screws just about disolved when i touched them, and rather than drill them out a chase threads, a rebuilt TBI with new IAC and TPS cost about the sum off all the parts, so I took the easy route. After the fuel pump, I thought maybe the reman throttle body had a bad regulator, so I tried putting my old unit back on, but the very rich over fueling symptom continued.

In order to make this look more like a forum post than a novel, im going to bullet point a few other thigs ive checked along the way.

-Engine temp sensor, ohmed, appeared to be good, replaced just in case
-Map sensor, checked for internal vaccum leak and ohmed, appeared good.
swapped with another good map on loan to check, no change
-Gasket underneath the TBI adapter to the manifold was replaced.

Any advice would be greatly apretiated, im loosing my mind with this one. I have checked a number of threads regarding TBI issuse, but I havnt come across one that mirrored my transition from lean before fuel pump to rich following. Unfortunately all the little thigs that occured between the initial issues and the fuel pump replacement have created more variables than I would have liked to, but it it what it is now.

-Ryan