For almost a decade, my 89 245 has had a rough idle. The choppiness seems to vary from moment to moment, but gets worse when the engine RPM settles down to idle speed (around 800). I still have the original 561 ECU in the car--could this be the source of the problem, or is ECU failure an all or nothing proposition? Could installing the newer 951 ECU offer performance improvements?

OR, do I just have consistently dirty fuel injectors (even after running on 93 octane fuel for years)?

(BTW, The usual other suspects have been ruled out--engine out of tune, dirty throttle body, bad sparks, vacuum leaks, etc.)

Thanks.

Clean the throtle body, IAC valve (motor), flame trap and all associated hoses abd vacuum lines. Also pick up a spare ECU, AMM and fuel pump relay before you really need one.

My '84 (195K miles) had a lousy, really lumpy idle, for quite some time. Couldn't fix it, although I checked what I thought were all the usual suspects (even the AMM, by temporarily disconnecting it). But...
one day, the AMM died (wouldn't idle at all, unless I disconnected it), and I replaced it. As soon as I put in the new AMM, the car idled -- purred like a kitten, and it's been rock steady ever since. I feel that the old AMM was faulty all along.
My point is that while the usual test (try disconnecting it) for a bad AMM is good for diagnosing a completely dead unit, it is not sufficient for a mere dying AMM; and (in from my experience) a bad AMM can cause your problem.

BTW, The usual other suspects have been ruled out--engine out of tune, dirty throttle body, bad sparks, vacuum leaks, etc.

Well, what falls under the heading of "etc?" Spark plug wires? TPS adjustment? IAC functioning? Injectors/seals? Engine mounts? All of these could cause rough idle.

And how do you know for sure you have no vacuum leaks? I mean, the Pope has vacuum leaks....

I agree this doesn't really sound like your ECU, but it couldn't hurt to have a spare from the junkyard and swap it out.

My rough idle was caused my a multitude of troubles. Fixing things like severly worn control arm bushings, bad struts, plugged up brass nipple into trottle body, faulty CPS Sensor, etc. I think you should look elsewhere for your rough idle.

I do not think so and I would not suggest it, although you should have a cheap spare on hand to swap. Beats me how some guys drive around without a spare 561/951 and an AMM. fuel relay, etc. But I actually like junkyards.

Bob,

I picked up a spare ecu for my '89 760 TI. Same part number, but memory fails me at the moment as to what the number is. There is a two pin connector and separate jumper on the harness right near the main connector for the ecu. The one installed in the car has the jumper installed on the connector and the one I pulled from the junk yard has the jumper hanging loose.

Do you know the purpose of this jumper, and more importantly, should it be installed during normal operation?

Thanks,

Dan

Same situation in the 240 harness of '88 with an extra connection to something (I should know but it slips my mind) and the 85-88 harness with the jumper. Otherwise identical, as you would expect within the LH 2.2 family.

I had an 86 240 that had a lumpy idle for years. Replaced the FPR and used injector cleaner. No dice. Finally, I bought a set of used injectors and put new seals on them and swapped it out! Seems to have solved the problem, as the car runs and idles better. However, I have had 89-90 wagons and gone through the bad ecu deal! The 951s are harder to find. IF you find one cheap, it might be worth it - I have found two on ebay in the past under $100.

Bob Weber
Hamilton, Ohio 45013
59 Volvos since 1979