420SEL idle level
 

I recently drove my daughter's '88 420SEL and noticed that the idle went up to a steady 1,000 RPM when stopped in Park, but was steady at 600 RPM when stopped in Drive with the brake on (like at a stop light).

The car runs fine otherwise -- is the out-of-gear 1,000 RPM idle level anything to be concerned about?

THE IDLE IS SUPPOSED TO BE MODESTLY HIGHER IN NEUTRAL OR PARK. THE IDLE CONTROL IS CONNECTED TO THE NEUTRAL SAFTY SWITCH FOR THIS INPUT.
SINCE THIS CAR IS AN AUTOMATIC THE TACH. IS REALLY FOR ENTERTAINMENT,HAVE YOUR TACH. CHECKED AGAINST AN ELECTRONIC SHOP TACH. AND YOU WILL FIND THEY ARE OFF BY 100RPM OR MORE(TYPICAL)
MF

I have found that the idle control valve is usually at fault. Its expensive to replace.

Where is the idel control valve on the 420?

In connection with replacement of the 420's radiator (hairline crack in top plastic), I had the idle control valve replaced -- was indeed expensive: $330 part [I let my idependent supply and charge list as I have frequently provided my own parts for other renewals] and $65 labor.

Has cured the idle irregularity.

Its directly in front of the throttle body and behind the water pump on top of the engine.

I have experienced a similar problem with my '86 420 SEL but it usually occurs after a prolonged driving at highway speed. I was able to verify that it was indeed the idle control valve by giving it a tap of a wrench with the engine running...it then settled down to "normal" (around 600 RPM). My mechanic said that the bugger can be cleaned out...I wonder how that's done...sure would prefer that to spending $300+!!!

Our high idle was persistent -- stable 600 RPM in gear stopped with brake on and steady 1100 RPM in park or neutral -- didn't seem related to any driving conditions.

My independent is very reliable and doesn't do additional work unless needed, so I assume that they checked the valve and found it needed replacement (I picked the car up after hours and haven't had the chance to discus it with them).

Our car was a low mileage (75,000 on an '88) grandparent's car which I'm going ahead and refurbishing all items as they come up since in addition to its being my 16-year old daughter's daily driver (high priority on reliability and safety), we use it for long distance family trips to keep the mileage down on our newer leased cars (like my C43 AMG) -- I'm willing to pay the price for certainty of reliability in such a solid, roomy cruiser -- they don't build 'em like the old 126s (tho I may trade up for a '98 or '99 140 in a couple of years).