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Old 02-15-2003, 08:03 PM
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Join Date: Nov 2001
Location: west orange, NJ
Posts: 47
high idle still - stumped

My 91 420 SEL idles at 1800-2000 rpm in park/neutral when warm and 850 rpm in drive. When cold, it still idles high (above 1000 rpm). I've cleaned and checked the operation of the idle controil valve which seems ok. I swapped out the idle control module with a spare from my local tech but still no change. The module is getting power. When hot or cold, the duty cycle on the idle control valve is at 70% . It should be near 35% when warm. This makes me think something is telling the circuit that the engine is cold and is holding the valve open but the temp sensor checks out ok as well. I'm stumped. Any thoughts anyone?


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Old 02-15-2003, 09:52 PM
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Join Date: Jul 2000
Posts: 638
high idle

there are 2 temp sensors I believe. One for the idle speed control, the other for other engine management functions. Check them all.
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Old 02-16-2003, 01:49 PM
Senior Member
Join Date: Sep 2002
Posts: 3,062
1,800-2,000? That's around the same as unplugging the idle air valve. I can't say for sure what's up here, but I would check ALL fuses and ground points. There's a ground point at the top of the engine, on the intake manifold? driver's side? I think? It's a couple brown wires connected to the bracket? I apologize for the sketchy information, but I don't have access to MB info at home. Anyway, take a look around (with the air cleaner removed) and you'll see it. I've seen this come loose and cause some strange behavior! Good luck
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Old 02-16-2003, 04:05 PM
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spray carburetor cleaner on each injector and see if the idel increase further. If so replace the injector mounts and seal and the idle air compensator will begin to work properly. If it is like mine the vacuum leaks around the old mout seal created a demand for fuel and the idel is sped up by the compensator trying to get the proper mixture. To further confirm see if your mileage has dropped significantly.
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Old 02-16-2003, 04:34 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,318
If it's like the 380, there is a coolant thermo switch on the back of the engine. If you pull off the connector and bridge the contacts, you are simulating cold running.

Again on the 380, the lambda is controlled by an oil temperature sensor.

But I agree that with an idle that high you are probably not getting any voltage to the valve, or the valve is stuck open.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 02-16-2003, 10:23 PM
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Join Date: Dec 2001
Posts: 577
J. Chip - you may have seen this post. Who knows, it might be worth a look?

Fast Idle Problem Cured!
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Old 02-17-2003, 02:47 PM
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Join Date: Nov 2001
Location: west orange, NJ
Posts: 47
Thanks for all the input guys, however....

I've checked the 4 pin dual temp sensor and it checks out as per the temp vs res table.

I am getting power to the N8 module and about 4 volts (with a 70% duty cycle) to the IC valve so I don't think my problem is related to shorts or grounds.

While I do expect there is a vacuum leak somewhere, I have been all over the engine with carb spray and can't find a problem.

System is quite a bit differant than the 380.

Still stumped.
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Old 02-17-2003, 04:31 PM
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you should measure millamp on the valve and if too high pull valve in the center remove plastic plug screw in screw into brass piece in and slowly pull out very little at a time till you obtain proper milleamp reading i will send them to you tomorrow when i get to shop.
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Old 02-17-2003, 04:33 PM
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only do this if sensors and everything else is correct

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