[W126 Coupe] Slightly high Idle- What next?

[Tim Graney]

First thing to check is that the linkage has not been adjusted to raise
the idle. Disconnect the linkage right at the throttle plate and see if
the idle goes down to where it should be. If it does just lengthen the
rod to fit. Secondly, check for vacuum leaks. Put a vacuum gauge on a
manifold source. There should be three sources. One on each side of the
manifold and one in the rear of the manifold. Look for even readings of
20 bar, needle should not fluctuate at all if it does there is a leak or
leaks. 
 
Any additional air coming into the system will increase the idle. Leaks
can be anywhere. The biggest culprit are the rubber O ring gaskets that
connect the upper and lower pieces of the intake manifold. If you have a
Mighty Vac you can check for leaks yourself. Check all the vacuum
connections for cracks and loose connections. Check the rubber
connectors on the idle control valve for tightness, they become dry and
brittle with age and loosen up creating leaks. There are a myriad of
connections behind the dashboard that control the HVAC system. Be
patient and eliminate each connection as you go along.  The vacuum gauge
is your friend and they are cheap. You can tune your car with one of
these. 
 
 It sounds like your idle control valve is OK. The unit itself might be
bad, there is no way of knowing without the proper equipment to test the
unit. If you buy a new or rebuilt unit make sure you get the unit that
matches your vin#  there are differences. 
 
The CIS system is very competent but also very mechanically complicated.
When fresh the system is great but it tends to become problematic as it
gets older. A great Benz Mechanic once told me years ago that it is not
the mechanical systems that fail on Benzes it is the organic components,
i.e.. rubber connections and gaskets. 
 
Hope this helps out
 
Tim Graney
1959 190sl
1965 250 se coupe
1961 300d Adenauer
1985 500 sec amg
 
 ----Original Message-----
From: mbcoupes-bounces at mbcoupes.com
[mailto:mbcoupes-bounces at mbcoupes.com] On Behalf Of
JohnJayKimrey1 at aol.com
Sent: Monday, June 25, 2007 1:56 PM
To: mbcoupes at mbcoupes.com
Subject: [W126 Coupe] Slightly high Idle- What next?




Hello everyone:
I know most of you have heard enough on this subject but I am not sure
what to do next and any and all advice and information I can get is
appreciated .I have an idle of 850 rpm in park or drive with or without
a/c. It is smooth and doesn't change after initial warm up. I have
checked for vacuum leaks and found none. Air metering plate moves freely
and closes without a problem. OVP relay is new(previously sticking and
ABS light was on and rpm 1100).No binding of throttle linkage and
throttle valve closes freely. Throttle valve switch checked good for
both open and closed position with ohm meter, Idle control valve(ICV)
appears to be operating but cannot tell if it is closing completely.
When disconnected idle goes to about 1800 rpm and connected to 850 rpm.
Resistance is 4.5 ohms and voltage across connector is about 5 volts+-.2
volts when connected and at idle.The valve is moving freely inside as I
removed and checked it. Thermo time switch and cold start injector are
working as idle dropped to 850 from 1100 after  about 30 seconds of
start up but not to 550-650.Is there anything I may have missed and does
my problem now direct me toward buying an expensive idle control valve
and or idle control unit. I don't have access to either units to swap
and no MB mechanic in town. Any advice is appreciated. Oh yes, my
vehicle is a 89 SEC with 80K miles and records show scheduled servicing.
I bought the car in May 2007. It had the high idle problem when
purchased.
Regards,
John K.




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