1990 mercedes sec 500 m117 intermittent high idle

dannysec

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Joined
Apr 28, 2013
Messages
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Age
32
Location
colchester, essex, uk
Your Mercedes
mercedes 500 sec w126
I having a problem with the common high idle!, but with my case it randomly settles down and idles correctly doesnt matter if i have started it cold or warm it is completely random! it idles from 1100 rpm to 1300 rpm. eg I get up from work start the car in the morning has a high idle pull up at work idle has settled down. or pull up at one traffic light and its idleing fine and then the next its high again :S could this be cause but back pressure of the exhausts?

HELPP
 

robparker

Senior Member
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Joined
Apr 18, 2006
Messages
2,695
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426
Location
Taunton
Your Mercedes
220d (115), 190d (201), c63 (204), 220ce (124)
prehaps try lubricating your complicated throttle linkage to begin with. Otherwise its pretty much always the relay in the passenger footwell marked 'idle speed'
 

ianrandom

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Aug 9, 2009
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Location
Berks & South Devon.
Your Mercedes
1986 420SE, 1990 420SEC, 1989 2cv6.
The relay, or the idle valve - the valve can be soaked in petrol, brake fluid or similar for a few days and that can help free it up.
If you know anyone good with a solder gun the relay could be free touched. Similar with the OVP relay, the one with the fuse in it between the two fire walls. That can get intermittent.
 

television

Always remembered RIP
Joined
Mar 14, 2005
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Age
87
Location
Daventry
Your Mercedes
2002 SL500, 216 CL500, all fully loaded
You must check the switch on the throttle body, this switch switches over from the idle ECU to the main ECU, in the event of a fault in the idle circuit it switches to the fail safe high idle. When running on the idle ECU either putting the air con on or engaging D should increase the idle by 50 rpm, if there is no change when doing this its running on the main ECU.

Fluctuating RPM can be vacuum hoses, max vac at idle and finds a leak, more air goes in and the engine speeds up, as the engine speeds up the vacuum decreases and the speed drop, then the process starts again
 

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