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Big post, hopefully all this info will be beneficial -

Figured I'd post here for more ideas before I start pulling the car apart tonight. I've already gotten some great advise from Captain, another user on here, and I'll go over everything that's been going on and what I've already tried:

Bought a 1991 manual turbo with 106k miles about two years ago with a thrown rod and blown turbo. Took my dad and I over two years to finish rebuilding it and getting it legal. The engine itself is almost entirely stock save for some intake piping and an HKS bov. On the first few attempts to start it after the rebuild, it wouldn't idle and would seem to bog when accelerating. I replaced the AFM and it was fixed for a while. I drove it for a couple weeks daily throughout January and the start of this month and it was running like a champ, but I did notice that sometimes it would just die when coming to a stop in neutral. No indication, just went from 1500rpm (as an example) to 0 while braking with no accelerator and in neutral. It was easy enough to restart but it progressed to the point where I now have a code 31 and it wont idle at all. I know it's not the AFM, as I've now tried 3 of them and nothing has changed (even after ECU resets each time). It's done this a few times in the past and I would just remove the AFM, clean it (with compressed air, never any solvents) and it would be good as new. Not this time.

I have a new TPS and IACV that I'm going to test over the next couple days. I also pulled the new cone filter off, which I may have overoiled, and cleaned it and let it dry. Before pulling it apart to replace the TPS and IACV, I have some carb cleaner I'm going to use to test for vacuum leaks and just go over the entire car for loose hoses. I'm also dropping the ECU to make sure there's no loose plugs or issues there.

I've already tested a different BOV to see if mine was stuck open and creating a giant vacuum leak, but that wasn't it either. I'm also going to test the timing and coil packs, CPS, etc.

Anyone have any other recommendations for things to check? I figure if I'm going to have the car apart anyways I might as well check every possible cause.

More info on the behavior that led me to this point:
Occasionally when coming to a stop, the car would die. This seemed entirely random at first. As days progressed, it would get more frequent and start showing signs of it happening before it stalled. For example, sitting at a red light for a minute or more, I would notice the rpms start to slowly drop from around 750 down to 600, 500, 400 then it would either die or I would give it a little gas and the issue would be gone. Again, as days progressed, giving it gas would no longer fix the issue for the timebeing, it would raise the rpm back up to about 750 and then it would repeat the process until it finally did die or I was able to leave the red light. The entire time it's below 600rpm or so, there would be a CEL. This CEL would turn off the moment I'm over 2000rpm exactly.

At this point, I can turn the car on, it'll idle high as it warms up (20-30 seconds), then it'll slowly drop to around 750 where it should be, but then it'll bog and stall immediately.

Hopefully all this info gives someone some ideas! As I mentioned, I've already gotten some great help and I have a list of things to test but more ideas is never a bad thing. Thanks in advance for any help!

TLDR; car wont idle, code 31. already replaced AFM, what else could it be?
 
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