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Old 11-17-2002, 11:18 AM
Registered User
Join Date: Nov 2000
Location: Gaithersburg, Md
Posts: 40
300E idle adjust

I recently dropped a 300E engine into my 260E and I now have two problems that I can't seem to figure out. The first problem is that the engine idle is REALLY low (about 500rpm in neutral). I took off the air cleaner housing and saw a black knob which I assume is the idle control knob but I'm not sure. I don't want to mess with anything until I get some advice from a professional. I think that the idle just needs to be raised using the knob. The second problem I have is that the fan stays on, no matter what the temperature is so for now I just disconnected it. Any help with these problems would be GREATLY appreciated, thanks.
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Old 11-17-2002, 12:24 PM
Registered User
Join Date: Jul 2002
Location: Stony Brook, NY
Posts: 160
I have an 88 300SE with the same engine and the black knob in the middle of my cleaner according to the mech is a fuel richment adjuster. Assuming were speaking of the same knob. The one I'm speaking of is actrually accessable with the cleaner in place. There's a "hole" in the cover that allows you to place a longish screwdriver with the cleaner in place. I don't believe this affects idle speed just fuel richness with no air adjustment. I'll keep an eye on this thread as I'd like to raise my idle a touch as well.
Ron Brooks
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Old 11-17-2002, 01:18 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
No idle adjustment, the system is self regulating. The system has to work right to be right. If the car is a 86-89 there is an electrical solution that adds about 50rpms.

Does the car idle the same speed with the idle air valve unplugged? If so my guess is that its stuck. The idle valve in the unpowered position runs the car at a small bit faster than spec idling in neutral with the A/C off. Normally this would be maybe 700rpm with the electronically corrected value being 600rpm. When the unplugged system has the car placed in gear the constant amount of air is insufficient to maintain the idle under the new load and could drop to say 500rpm. The electronically corrected system will open the valve and stay at 600rpm.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 11-17-2002, 06:29 PM
Registered User
Join Date: Nov 2000
Location: Gaithersburg, Md
Posts: 40
I don't know where the idle air valve is but if you tell me where it's located I can check. By the way, when I had the new engine put in a mechanic was playing with the black knob I mentioned earlier and it raised the idle alot and he played with it again and now its just too low, I'm not sure what year the engine is from either. Thanks for the help.
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