I heard when computers go bad in these it's generally the capacitors on the PCB. Which can slowly get worse until finally they just can't give the computer the voltage that it needs. Causing it to do all sorts of weird stuff. (that's how I understand it anyways) happened to an old desktop I own, I replaced the caps on the motherboard, and it's good as new.
I know the EEC-IV isn't very powerful at all, but I imagine it's similar in more ways than not.
I'm leaning towards it being the computer. You're right, a lot of wiring doesn't just instantly go wack, otherwise it would probably be quite obvious.
Maybe where the main harness connects at the firewall?
Here's the thing... it may not be the computer. It could be a short and the computer is protecting itself by disabling features in the short circuit. When you reconnect the battery, it wipes memory and starts fresh. When you run the test, it realizes something is broken and shuts down into limp mode. The last thing you want to do is get a new computer and risk frying it. You need to start doing wire tests. Since it ran better with the IAC unplugged, maybe the short is in that area of the harness.
That does make a lot of sense. However, what about it spitting out all the other codes? Wouldn't it just spit out one 998 or whatever? I'm going to do some research!
Who knows how old those codes are is all I can say. :/
Plus it does only spit out one 998! Remember that the big bundle of codes I have is from KOEO.
The 998 is KOER, and it comes along with only one partner code.. 57.
So Naten could be right, but finding a short that all the sudden developed overnight is going to be a pita bread with hummus.
Oh, so you already cleared the codes you got from several posts back? And they didn't come back? So yeah, I would definitely start inspecting all the wires and connections you can.