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Old 03-01-2013, 09:58 PM
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Join Date: Oct 2012
Posts: 13
W124 - Fast idle stuck at 900rpm

My dad's 1991 300ce has a fast idle issue and I can't seem to solve it.

I do know the issue is related to the idle control valve. I have unplugged the ICV and the idle immediately drops to the normal 500 RPM. (This set off the check engine light.) The OVP fuses look good as well.

I went ahead and replaced the Idle control valve and it helped reduce the idle RPMs from 1500 to 900, but it still won't drop to a normal RPM and is a little hard to start.

I ran about 3/4 can of seafoam through the intake and put 1/4 can in the tank just to make sure nothing was blocking the intake. It seems to have fixed a slightly rough idle, but the RPM remains at 900.

I will be replacing the temp sensor in the morning to see if that helps. The thermostat is relatively new so I'm not planning to replace that.

Anybody have ideas what to check if the temperature sensor doesn't work?

Last edited by SchnellSchweitz; 03-02-2013 at 03:12 PM.
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Old 03-02-2013, 01:54 PM
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Join Date: Oct 2012
Posts: 13
So I checked the ICV plug and I'm only getting 2.6 volts.

Is this normal???
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Old 03-02-2013, 05:32 PM
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Join Date: Oct 2012
Posts: 13
Latest update.

I tried a different OVP just for fun and no change. I removed and reinstalled several vacuum hoses and sprayed a little silicone on the throttle linkage and now it "intermittently" settles at 500 RPM.

So I must have done something to the system, but I'm still stumped as to why it has improved.

My worry is that it still sticks at 900+ RPM occasionally.
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Old 03-02-2013, 06:47 PM
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Join Date: Jul 2009
Location: cleveland
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Did you look at the idle switch? Sometimes it looks ok, but the wheel part gets a chunk out of it and wont engage fully

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Old 03-02-2013, 08:26 PM
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Join Date: Oct 2012
Posts: 13
Ok... consider this one solved. It was the idle micro switch and throttle linkage as Lorainfurniture says above.

I just sprayed the linkage down with some WD40 and cleaned it off with a rag. I removed the micro switch and did a little cleaning to it with a paper towel and some WD40 at the switch end and on the contacts.

Slipped it back into place and she idles at 500 RPMs now 100% of the time.

Thanks to everyone's help. I do appreciate it!
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Old 03-05-2013, 12:39 AM
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Join Date: Jun 2006
Location: Chandler, Arizona
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Goodjob finding the culprit.

FWIW..500 is a little low...Correct OE spec is 650rpm +-50.
Check the idle control hoses and if they are rock solid, replace them to avoid future potential roadside issues. They are incredibly important hoses and eventually crack causing massive vacuum leaks. A 1/4in tear will cause a non-running condition.
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