Question about Mercedes-Benz 400-Class

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I asked you yesterday about replacing a radiator on a 1986 mercedes 420 sel. your info was great. one more question. engine is idling high. any ideas?

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1986 models have less control by computer of idle mixture than newer ones. I would look for more mechanical solution such as idle stop screw or carb idle setting if not fuelinjected. Not sure for this year which it is. Also vacuum leaks or air leaks can cause this so look visually for deteriorated lines or air tubes leading to the main air supply. Hope this helps and good luck

Posted on Apr 29, 2010

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  • andreozzihal May 19, 2011

    1986 420 sel besides the basics mentioned there r three major idle controls, one being the Overvoltage Relay located on drivers side firewall red cap 10 amp fuse, if fuse is not blown pull the whole unit out see if idle changes if not relay itself is bad. Next ICV (Idle control valve) located on the front of the intake manifold (two prong electrical plug connector,unplug connector see if idle changes,if not ICV is bad, finally the idle speed control module,location is under passenger side floor cover/rug on fire floor wall, unplug it if idle does not change it is bad! GOOD LUCK 1986 420 sel.

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There could be a vacuum leak. All vacuum hoses should be inspected and replaced as necessary. The computer does control timing but can be manually set. This could be an issue, or even someone messing with the CO adjustnment through the Mass Air Flow sensor. This is a very sensitive adjustment and should be performed by someone who knows how to do it. Another place to get a vacuum leak that is common is under the upper plenum(intake). Let me know if you find anything and if not we can go a little further into controls. You can email me at iambanshee74@yahoo.com and I should be able to find diagrams

Posted on Apr 29, 2010

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  • Kori Policy
    Kori Policy Apr 29, 2010

    I really enjoy these cars myself and have had a couple plus working on these for years, you learn a few tricks.

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