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This post is for all those who like to hear the punch line ... my problem is resolved, and since I posted a number of times on my way to this point, I thought I would share with those who helped, and those who might encounter a similar -- VERY frustrating -- problem in the future.
Here's a summary of the problem (using excerpts from one of my posts): I had an on-going "issue" (multiple issue) with my 16 valve 1989 740 (B234F). First I was getting a consistent check engine light and several symptoms: bad mileage (~10 MPG), exhaust smell, rough idle, black exhaust on hard exceleration. O2 sensor was replaced, check engine light returned, along with symptoms. Next, ECT was diagnosed as shot and was replaced along with thermostat. TPS and fuel pressure both checked out fine. The mechanic at the dealership told me I needed both fuel and ignition ECMs, which I didn't do because he wanted $2000. Mileage improved a bit (~12 MPG), but after a while check engine light returned, and idle got rougher than ever (~600 RPM). Started stalling when engine got warm and I was stopped at lights, etc. Throttle body was clean. Pulled codes 2-3-2, 2-1-3, 3-1-2 which would return immediately on restart after clearing.
It was due for a 30K service (211,000 miles on the car), so I had that done. Idle motor was replaced but the car was STILL running rich and stalling out. Also it appeared that the (new) 02 sensor wasn't working, so pulled that out and had it replaced with a Volvo full-harness part (originally had the Bosch universal part with spliced harness put in). STILL was running rich, stalling, etc.
Telling myself, and promising my wife, that this would be the last part I threw at it, I picked up a fuel ecu (Bosch #0-028-000-571) from a junk yard. Plugged it in and VIOLA ... the car runs like a top. Cleared the codes, and it's been 1-1-1 ever since.
So the bottom line -- in my case -- is that it was the computer. Did I throw some extra parts at it ... probably, but who knows. Replacing the computer in the first place probably would have got it running BETTER, but now it runs PERFECTLY. BTW, I've had a 2-3-2 in there since I bought the car used 20K ago, so some of the problems where there before the computer crapped out and the really bad driveablity issues started.
I got lucky and found a junked ecu locally for $150, but they're hard to find for the 16 valve engine (I called all over the country), and so it can be expensive to start there if you've got a similar problem. In any event, I think I probably tried everything you can for this set of symptoms, so if my experience helps someone else, I'm glad. Thanks to all,
Tom
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