My wife found a 1990 240 (California car with EGR) with 19,000 miles last year. We had owned a 1992 Volvo years ago which died in a crash. So, we bought it although I was pushing for a pre-owned BMW.
I'll shorten the story. It's remaining problem is a mild uneven idle. My indy Volvo mechanic says it's fine and stop comparing to my 12 year old BMW with inline 6 that has an extremely quiet and smooth idle.
I read about ECUs on this forum or another one. One writer said that he does not take long trips without a spare ECU in the trunk. He mentioned that these can go bad based on time not mileage. The installed ECU is a 556. I purchased a 933 from e-bay for a spare.
The 933 has a fuel pump prime noise when the key is turned on, the 556 does not. So there is a instant start with the 933 as opposed to a 1-1.5 sec crank on the 556. The idle is noticeable higher on the 933 so much that the car will take off from a stoplight simply with the foot off the brake. I measured it about 750. My tachymeter needle fluctuates becuase of the uneven idle. Otherwise, the 933 worked fine. I put the 556 back in since I like the idle better. Of note, the uneven idle was still there with the 933. I suppose I could have left the 933 in longer to let it adapt, then adjust the idle if it didn't drop.
At any rate, I feel I have an adequate spare. I don't have a specific question other than wanting to provide my observations on some differences that I noted between the two.

 

I've never had to replace an ECU in well over 1/2 million miles in 6-7 different volvos

so I dunno.

re: your uneven idle, it SHOULD idle around 700rpm. if everything else is OK and you have crummy idle, clean the throttle body, clean the idle air controller, verify the IAC works correctly (DTM-3 on the diagnostic box cycles the IAC along with other things), verify the IAC rubber hoses are in good shape, reinstall, check all the other vacuum hoses.

with the IAC unplugged but connectedm the engine should barely idle, way down around 100-200rpm. there's an air bypass screw on the throttle body you can adjust to bring this into range. the IAC shoudl bring this up to 700rpm or so, and if you turn on the AC compressor, it shoudl go up to 800 or so.

 

Thanks pierce. It's always good to get other perspectives on parts (ECU in this example).
I have cleaned the TB, IAC twice, checked other vacuum lines, but have not run the DTM-3 cycle. Will do.

 

what I think I'd do would be to disconnect one of the hoses to the IAC, and hook up a clean hose to it that you can gently blow into. run the DTM3 test in jumper position 2, and when the IAC part of the test cycles, blow into the IAC and you should feel it open/close/open/close quite crisply as it cycles on/off/on/off. if you do, then its good, and the problem is elsewhere.