My automatic 1989 240DL sedan's idle surges after coming off of the interstate, but only when I put my car in park or in neutral (1100-1400rpm). However, while my car is in gear, it'll remain normal (800-900). If I turn off my car and restart it less than a minute later, the idle will be normal again. Daily driving results in normal idle.

I visited a salvage yard and pulled a IAC from a '90 240, soaked/cleaned all of it but the electrical connector with carb cleaner and then installed it. Unfortunately, it did not fix the problem. The throttle body was cleaned about 6 months ago and still looks pretty clean.

Does anyone have any suggestions?

Thanks,
Eric

Eric,

This has not yet happened to me. I have an '89 too, and have been reading posts for three years describing symptoms of ECU failure some say is very common with this model.

If you search the archives you should be able to find some of the references to failing -556 and -951 replacements, "rebuilds" including new eproms and other black box magic associated with 1989 & 1990 cars.

Limiting the symptom to when you come off the interstate makes me think of heat soak in the ECU, imagining the transistor driving the injectors to be dissipating the most heat then, but the impression I get of the failure reputation and rebuild fix is that the control electronics are affected by heat.

The reason it has not happened to me yet is I have a spare pulled from the junkyard. Having a spare of anything prevents all future failures.

I also have a spare crank angle sensor...

Brickboard FAQ ECU Section
--
Art Benstein near Baltimore

Having had my throttle body cleaned and the TPS swapped out with another used (but in working condition) unit, the problem still exists so I'm betting on the ECU.

My gas mileage is fine, daily driving is fine, highway driving is fine until I shift into neutral or park, but like I said, turn it off and then back on, and it's normal. I think I'll just ride this one out.

Thanks for your help, everyone..

Eric

My car was doing the same thing recently, but to a lesser degree. I too had done the throttle body within the last few months - but found that a small amount of build-up had formed on the throttle plate since then. Cleaning it up seems to have solved the problem.

I'm bringing my car in for an inner tie-rod replacement, and while there, I'll just let my mechanic clean the throttle body and inspect it. Hopefully, this will solve the problem, and if not, then I'll consider replacing the part. I'm just hoping that something some grime has built-up on the unit so that this will have solved the problem.

Eric

My car was doing the same thing recently, but to a lesser degree. I too had done the throttle body within the last few months - but found that a small amount of build-up had formed on the throttle plate since then. Cleaning it up seems to have solved the problem.

I had a similar problem with my 745: it turned out to be a failed TPS (throttle position sensor) which was not sensing the "idle" signal when the throttle was closed. It was especially pronounced when cold and in neutral or park. Replacing the switch solved the problem.