hello all. i had posted this in the general forum area but havnt recieveda response in over a month. so hear we go again. 86 sel 420. it idles at an astonoshing 1500 rpm when hot in park. once it slams into gear it drops to about 1000-1100. dosnt seem to be as bad when vehcle is cold. if you have any recommendations or have experienced this please let me know. much thanks!

 

My 1987 420SEL does the same thing when cold. It idles high and then I let it warm up for about a min. and it drops down to normal rps.

 

Numerous things can cause high idle.
My 560 did the same thing a while back right after I replaced the spark plugs wires (again as I thought).but was fixed by my mechanic in germany (which is priceless to have) saidthe idle was set too high.Hmm. Apparently the last guy in america I took it toa few months before shipping it to America took theold spark plugs out, readjusted the spark plug by banging on them and then put them back (and on a side note, overtightening them) then charged me for sparkplug replacement while all he did was to adjust the idle to get rid of the evidence. Bugger.

Another common thing is the rubber seals under the mass air flow meterget dry and shrink causing air leaks and high idle. If ya handy with a screw drive or two, you can easily take it off and check. The rubber seals are cheap as hell.

Another cause could be just the standard vaccuum leak which can drive you up a wall to find sometimes.

Another mechanic told me that a slightly high idle rpm is normal when cold. Some say it could be a sign the idle control valve has one wire in the grave.