I havn't looked to far into this. I did hook up the Snap-On tester and all it said was wrong was the IAV but when I look at the valve with the eng running it is working. I think it is setting the coad because the engine won't stay at a even RPM. I did notice that the timing was going up and down also trying to find a place that the car would run at. Some times the car will smooth out and run good but rich. It will miss and backfire and run good a bit and start missing and backfire again. I know I must have a over full problem because of how rich it is running and I think that is why it misses and backfires. What would be the most likely place to look when the scanner said all was OK except the IAC and that is only active when the engine is running rough. When it is running rough you can take if off idle and it still runs bad. I'm going to put in a new set of plugs just to help it fire better with the over fuel. I wouldn't think the O2 would let it run THAT rich and not set a falt?? I watched the TPS on the scanner and it was at 4% at idle and would go up smooth to 100% I did the test last week and today was the first I got back in the car and yes nothing changed. Any help would be great.

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85 GT 4spd 2.8 1 yr blk
86 SE 4spd 2.8 2 yr blk
86 GT Auto 2.8 new to me.
87 SE 5spd Future 4.9 6yr

Your timing is bouncing? Sounds like an ign module to me. Also, whats the o2 readings look like on your scan tool?

I didn't look at the O2 because I didn't know what to look for. It had a reading but don't remember what it was. What should I be looking for??
Will the ign. module let it run rich?? I will hook up the scanner up again tomorrow and see what it reads. The car went through a 1/4 tank of gas in no time.

I am guessing that since you said IAC you are trying to run the 7730 ecm and not the caddy 2240 ecm? The caddy uses ISC not IAC. Some more detail about your configuration would be helpful but it almost sounds to me like there is a wiring issue.
Could be lots of things.
O2 readings on a scantool - median is about .500mv I think - less is lean and more is rich. .200mv would be very lean and .800mv would be very rich. Under normal operation its gonna be cycling so quick it will be hard to tell so those should help you 'ballpark' what its doing.

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Was this car running well at one point or is it a new swap?

I had a full day with corp. and didn't get the chance to look at the car again. Ya it ran good for about a week. Yes it is the caddy computer I just didn't word it right. It was the ISC, I looked in the book with the scanner and remembered reading that part. I plan on hooking it up again tomorrow after I change the plugs.