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  #1  
Old 11-24-2010, 04:06 PM
LandYaghtLover's Avatar
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Location: Green Bay, WI
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W126 560 idle seems high.

I just did some work, but first let me say when cold (at about 20 degress out) the car will idle at about 1000 rpm and may back down to 800 after it warms up. The car does not sound like its idling high (but it is higher after the work) and it does not want to pull away from me when letting off the brake. In drive it will hold about 800 rpm when stopped.

What I did was replace the two rubber pieces that meet the IAC (idle air control) valve. Or whatever Benz calls it. What I did not have was the little plastic tube that bypasses it to prevent a stall if the valve closes fully. So I used a barbed pipe that I cut to fit.

Two things may be the issue if idle is indeed high. The tube's inside diameter is too large and acting like a vacuum leak. Or I cleaned the valve and that did some damage. Looked like new inside when I was done with it.

So before I go crimping a perfectly good metal fitting to restrict flow, I would like some thoughts or info on where a healthy idle should be.

Thanks much!

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car
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  #2  
Old 12-07-2010, 11:05 AM
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Location: Indian Waters, Florida
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High idle

I just went through my '91 420 SEL. All the soft rubber bits had turned to hard plastic. The little connecting plastic tube on mine is acutally crimped shut on one end. It's made that way. I looked up the part no. on the Russian site then did a Google search for parts and sure enough it's suppose to be that way. On other models and years it is a hollow tube.
Mine was really running rough and surging at idle. I also removed the intake manifold, replaced the eight rubber connecting rings between the manifold halfs, and the big rubber piece that connects to the throttle body. All toast. Inside of intake had major carbon and coke deposits. Replace the fuel injectors, holders, and O-Rings. Replace all vacuum lines and rubber ends. Runs very smooth now, just idles high. About 1100 rpm.
Let me know if you solve your idle thing.

Dave
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  #3  
Old 12-07-2010, 03:53 PM
david s poole
 
Join Date: Mar 2005
Location: dallas
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i think that what you have left now with 420 is a worn out idle valve.replacing valve on my 420 cured the high idle.
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David S Poole
European Performance
Dallas, TX
4696880422

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
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Old 12-07-2010, 07:29 PM
LandYaghtLover's Avatar
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I too plan to do the same with the rings and that duck bill plastic thing. Just need to order and take a day off to remove it all and get the intake cleaned before reinstall.

But I did fit my idle. I crimped the tube to an almost closed position. I did so little by little. Was easy since all the rubber was new, so the idle control valve just came on and off without a fight. Eventually I got to an idle I liked. The tube is still open on mine, just not a whole lot.

The 560 is a bigger engine, so maybe it should be open. My understanding was that the tube was there to let some air through to prevent a full out stall incase the IAC reacts slowly.

Anyway, idle is at 800 now and everything seems ok. But I still have an irregular idle issues. Hence my wanting to replace the rubber under the intake next.

By the way, the vacuum trees to the injectors on mine were the worst. They had cracks all over!
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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car
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  #5  
Old 12-08-2010, 11:27 AM
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Quote:
Originally Posted by LandYaghtLover View Post
I too plan to do the same with the rings and that duck bill plastic thing. Just need to order and take a day off to remove it all and get the intake cleaned before reinstall.

But I did fit my idle. I crimped the tube to an almost closed position. I did so little by little. Was easy since all the rubber was new, so the idle control valve just came on and off without a fight. Eventually I got to an idle I liked. The tube is still open on mine, just not a whole lot.

The 560 is a bigger engine, so maybe it should be open. My understanding was that the tube was there to let some air through to prevent a full out stall incase the IAC reacts slowly.

Anyway, idle is at 800 now and everything seems ok. But I still have an irregular idle issues. Hence my wanting to replace the rubber under the intake next.

By the way, the vacuum trees to the injectors on mine were the worst. They had cracks all over!
Land:

You gave me an idea. I'm going to try the hollow tube. Just for giggles to see if it makes any difference. I'm the third owner. It may be wrong but, the books say its crimped.

Also, from experience. If you remove the intake replace every soft bit you can. You'll find all the soft black rubber has turned into hard black plastic.
Replace the vac lines and rubber ends going to the throttle valve. You can't get to them later. Remove and clean the throttle valve with carb. cleaner. Replace the gasket.
Delete the EGR system if you don't have emissions testing. It's the source of all the coke and crap you're going to find in the intake. I made an EGR tube cover plate out of 1/8" aluminum. Trace the EGR tube gasket for the pattern. Cut out using a jig say with metal cutting blade. Easy to work aluminum. On the exhaust manifold remove everything. Take the threaded adapter from the manifold and match the threads at a good hardware store. Install a hex bolt with thread sealer. Or, leave the fitting and screw on a 3/4" NPT pipe cap if you can't match up the threads the other way.
Remove the vacuum switch (red top) plug with a fine thread metric bolt. M10 1.0 thread pitch, ~3/4" long. Plug the "a" port on the throttle valve. Facing the firewall there are two small vac connetions one above the other. On the diagrams they 're marked "a" and "b" On mine it's the one on top. The "b" and the "c" (larger one on the side facing the passengers side) is for the fuel evap system. Use these.
Get ready to do some serious intake cleaning.
Any questions, parts numbers, stuff, gimme a shout here or PM.
Been there, done that.
Oh, take lots of pictures before you touch anything.

Dave
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  #6  
Old 12-08-2010, 01:12 PM
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Location: Indian Waters, Florida
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ICV

Quote:
Originally Posted by david s poole View Post
i think that what you have left now with 420 is a worn out idle valve.replacing valve on my 420 cured the high idle.
David:

Removed cleaned and tested the ICV. On car connected 12V jumper. Clicked.
Removed, cleaned until brown gravy stopped running out. Hooked up vacuum source and hit it with 12V. Cycles on and off, stopped vacuum.
Soooooooooooo..........I'm guessing its fuctions as designed or did, or am I missing something? I'm assuming it's just a solenoid acutated on/off valve.
Question? Is the little tube between your rubber elbows "crimped" or hollow? U: See post here. Says his (560SEL) was hollow and he choked it down to dial in his idle. MB refers to it as a Rivet. Item 402 on the diagram and MB Part No. N 910006 004019.

Thanks, Dave
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  #7  
Old 12-08-2010, 01:21 PM
LandYaghtLover's Avatar
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If the idle valve is like I have seen on most cars, it's variable. But I am not sure with MB.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car
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