Here's the story. First of all, my idle is too high, I want to make it lower, the current idle speed is near 900-1000RPMs, and I need it back down to the original idle RPMS of 650 or 700 or whatever it is, and second, when I let the car sit running sometimes, the engine will rev up. I can stop it sometimes by just tapping the accelerator pedal, and the revving will stop and go back down to idle. But now, it's started again, and when I tap the accelerator, the revving goes even higher, and I have to yank the key out fast to shut down. This is all in the PARK and sometimes in the Neutral gear by the way. When in D or R or any of the other gears, there's no problems. I fear it might be damaging the engine, and maybe the transmission because when I sometimes shift gears, the engine revs up real high, then goes back down to idle when the shift is complete, causing a big jerking sensation. I don't like this, what could be causing this? Is it a simple fix? It did this with the hood up once, and I saw no movement of the throttle linkage cable. Also, can I adjust the idle lower without doing anything major? This is on a OM617.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

How is the blowby? Old diesels used to runaway with themselves like that at idle, but under load it can't just take off. Since the oil is the motor's fuel the oil mist speeds up the motor which then makes more mist which speeds up the motor which....

I'm not certain on the idle adjustment on a W126 if nobody replys I'll look it up on a W123, I'd think it'd be quite similar.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

check your throttle linkage for proper adjustment, and nothing is interfearing with it.

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matt

That is your only problem. Anything above 1000rpm under no-load is not governed by the IP. If it goes above 1K (After the AC turning off for example) the IP won't control the amount of fuel injected, and soon you're at 5Krpm before you know it. Don't ask me why, I have no clue.

If you turn it down, even to 800-950 range your problem will completely go away. On the OM617 it is very easy to do. Look on the side of the IP, there should be a point (Only one point) that the linkage hits against. There should be an adjusting screw with a flathead screwdriver slot and a locknut at the base. With the engine on, loosen the locknut and back out the screw until it's where you want it. Easy as pie.

Anybody got a pic of where the IP is that I'm supposed to adjust? I need some direction here, for all I know, it may be the easiest thing to find :p
And what is the factory spec idle RPMs for the OM 617.951?
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

The spec is 750rpm +/- 50rpm. I have mine set at 800rpm as I do not have any engine shocks at all.

Here is a link to the correct way to adjust it.

http://skinnerbox.steaky.org/123_DISK2/program/Engine/617/07_1-100.pdf

The PDF file 0750.pdf actually page 0750.17/1 gives a clearer example of what to do and how to adjust the linkages after you've made the adjustments. A day back I attempted to adjust the idle on my 83 300TD. It was idling too slow, say 600 rpm, falling off to 500 and stalling when 32F outside when put into gear after starting.

As a warning what I discovered was that if you adjust the speed upwards the screw has a spring loaded end and because of how the linkages are if you don't don't detach push rod 70 as per instructions the spring will compress but not increase speed until you jiggle the linkages, then the linkages reset to their new placement as the spring uncompresses and the engine takes off... Most exciting, however I had it in gear at the time and manged to turn it off.

Oh and the manual clearly states:
"Caution
If speed is set higher, engine will leave idle speed control range and may increase to maximum engine speed (at no load)."

They aren't kidding...

Forgot to mention here is a good time to clean each linkage head/socket and lube with ATF. Rumor is this improves the ability of the cruise control to mantain a constant speed without seeking.

In my case the same symptoms ended-up being worn out engine mounts.

My engine would twist and pull the throttle cable with it. Causing the exact symptoms that you describe.

Changing my engine mounts fixed my problem.