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Old 08-24-2001, 09:55 AM
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Lightbulb How do you Idle down RPM's on '86' 420SEL?

How do you idle down the RPM's on an '86' 420SEL? When I start the car, it is idled at 11,000. I read in a book somewhere that it should be around 6,000. The car seems to burn allot of gas. I mean, I do realize this is a large and older sedan, but when I tried to figure the mileage per gallon of gas, it's awful. Where under this great big hood do I look?? Can it be idled down without taking it to a mechanic?
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Old 08-24-2001, 10:41 AM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,357

I assume you meant to say 1,100 rpm; not 11,000 rpm. You wouldn't want a 6,000 rpm idle either.

Check out the SEARCH facility. Key in FAST IDLE and only search the TITLE area for starters. There are many things that could cause a fast idle to include:

- faulty idle speed controller/actuator
- sticking throttle linkage
- TPS(throttle position sensor) out of adj.
- Vacuum leak
- Bad throttle switch
- Air plate position sensor

These a some of the possibilities.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 08-24-2001, 02:41 PM
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Wow, imagine a 6000 rpm idle!
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Old 08-25-2001, 12:07 AM
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Thanks! I kinda got carried away with all those zeros. I pulled a 12 hour tour and was kinda sorta dain bread. (brain dead)
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Old 08-25-2001, 07:53 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
As stated there isn't any adjustment! The idle speed computer matches the RPM imput to the engine speed. IF the idle speed stays around 1,100 then have the throttle switch checked out. This switch has a full load & idle contact. IF the switch doesn't close at idle then the speed will remain at 1,000 or higher. THIS is true only for 1981 to 1991 V-8 engines with CIS injection, like your car has.
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