M-103 high idle - PeachParts Mercedes-Benz Forum

400Eric · #1 11-30-2009, 05:43 AM

My 89 300E Automatic has developed this problem of slipping into high idle almost all the time. Anywhere from 1,200 to 1,500 RPM. No amount of gunning the gas pedal will make it go back down but I can get it to drop back down to normal (700 RPM) by quickly turning the engine off and then quickly back on again (before it dies) which leads me to believe that this is an electronic issue. In doing a search I found some folks talking about the OVP relay but I do not believe it is the OVP Relay because I don't have any other issues associated with a OVP failure. Thanks to all who help.
Regards, Eric

Madd78 · #2 11-30-2009, 08:05 PM

Quote:

Originally Posted by 400Eric

My 89 300E Automatic has developed this problem of slipping into high idle almost all the time. Anywhere from 1,200 to 1,500 RPM. No amount of gunning the gas pedal will make it go back down but I can get it to drop back down to normal (700 RPM) by quickly turning the engine off and then quickly back on again (before it dies) which leads me to believe that this is an electronic issue. In doing a search I found some folks talking about the OVP relay but I do not believe it is the OVP Relay because I don't have any other issues associated with a OVP failure. Thanks to all who help.
Regards, Eric

This is kinda funny, but read my post in ps2cho's article. It just could be....

http://www.peachparts.com/shopforum/showthread.php?t=249224

pawoSD · #3 11-30-2009, 08:35 PM

Might be a faulty idle control valve sticking open too......I'd check/clean it.

Vacuum leaks can also cause a high idle....check all the rubber parts and hoses.

400Eric · #4 12-01-2009, 05:04 AM

Thanks for responding guys.

Madd78, I had read that thread already but the thing is I don't have any other symptoms. The car runs great, has plenty of power (for a little 2 valve 3.0 liter anyway), and gets decent economy (at least for a car that always, constantly has gas flowing out of it's injectors). I'll keep that on the list of possibilities though.

PawoSD, idle control valve makes sense, but if that part has an issue, then why would that part always work right when I "reboot" the car? I'll have to check that one out though.
I don't think it's a vacuum leak, or any other mechanical issue because of the way it settles back down when I "reboot" it by turning it off and then back on again quickly.

Open to other thoughts, ideas, emotions too.
Regards, Eric

Nateuton · #5 12-01-2009, 10:13 PM

Quote:

Originally Posted by pawoSD

Might be a faulty idle control valve sticking open too......I'd check/clean it.

Vacuum leaks can also cause a high idle....check all the rubber parts and hoses.

I just want to add that the ICV gave me problems on my 91 w124 as well. It would not idle at all and died unless I kept the RPMs @ 2500 or greater. I took it off after tracing other leads to no avail, ie potentimeter (sp), injectors, fuel pump relay ect.. My ICV was sticking shut. Got it unstuck and ran power to it while it was on the bench and lubed it with teflon spray and my car has not had a problem since. FWIW.... might be worth looking at.

400Eric · #6 12-02-2009, 04:33 AM

I'm taking this car to the drags this Sat. cause I've been wanting to know for going on 5 years now what this car runs in the 1/4 mile. Just for grins, I'm gonna try my 88 300E's CIS-E ECU in the 89 (it has an earlier part number) to see if it makes the car any quicker. I'm then gonna just leave it installed for a while and see if the idle situation changes. I'll report what happens.
Regards, Eric

400Eric · #7 12-09-2009, 06:29 AM

As I said before, I wanted to try the earlier ECU to see if it'd make the car any quicker..... it didn't. But I left it in to see if it would solve this high idle problem and unbelievably it has so far. It's been in since Sat. and I haven't had a visit from the high idle monster yet. It could be that the weather has turned colder or just plugging and unplugging the ECUs restored a good connection, who knows. So I'm leaving this ECU in there for a while and I'll report back again in about a month or so.

To anyone thinking of doing some ECU swapping of their own amongst M-103 W-124s, I would advise you to keep the 86-89s in one group and the 90-93s in the other group and don't cross those groups.

BTW, the car's best E/T of the day was a 16.969 @ 82.49 MPH uncorrected which is about a half second slower than it ought to be but what do we expect from a 21 year old car with 231,000 plus miles? I'm gonna try to tweak the EHA though.
Regards, Eric

400Eric · #8 06-29-2010, 01:37 AM

Sorry, I'm about five months late with the promised follow up.

The high idle problem never came back. I eventually had to give B-2's ECU back to B-2 and install a different ECU procured from my favorite You-Pull-It. The replacement ECU had the added benefit of making B-1 a quicker car as well which I attribute to the fact that it is from a non-California car and thus is calibrated to federal emissions standards rather than California emissions. High idle problem still hasn't returned.
Regards, Eric