idle speed knob on 240D is NFG
 

The plastic knob has already broken off when I bought my 83 240D. Per the tach I recently installed, it is idling at 700 rpm from a cold start. Turning the broken knob with pliers does not change the idle. It seems to idle fine but it is still warm (50 to 60F has been the coldest) but it will get much colder in the teens. What is the purpose of the idle speed knob? Is it necessary? Should I try to fix it? Is it a PITA to fix?

It is there to adjust the idle a bit to smooth out the idle when you first start up in cold weather or if running the ac. You may never miss it. otoh you can surely buy the knob for not too much $. I let one go with the dash I sold a few months ago with the cable attached. They seem to be about 50 50 gone in the old 240s and euro 300s.

Tom W

You must press the accelerator and then turn the knob.

Really? I am trying to picture what the mechnism looks like.

The knob turns a screw-type mechanism that pulls the cable which limits how low the accelerator linkage, and therefore idle speed, can go.

Pressing the accelerator removes the pressure from the cable so that it can move without binding.

Hold on, I'll make a video real quick.

http://www.peachparts.com/Wikka/W123Evaporator

Picture # 50 shows the knob.

Unless the engine is running at 100%, IE new, it would be helpful in the winter.
Otherwise start the car when cold and slightly press the gas pedal for a higher idle until the engine runs smoothly.
The knob pulls off and silver ring is unscrewed to remove the cable from the dash. The cable is routed through the firewall and attaches to the throttle linkage on just above the injection pump where the stop letter is labeled.

Dave

HERE is what the cable does under the hood. About half way through the video I try to adjust the cable without pressing the accelerator first. As you can see, all it does is slightly flex the spring.

I agree you should push the peddle to adjust it, however mine will adjust pretty easily even with out removing the pressure.

Lance, that is on your 617?

Thanks Lance. Great video! I wasn't sure how that knob/cable was supposed to work. BTW, I never received a reply on the 300D on craigslist.

John

hum, it was sent out on the 20th.

Yes. My engine is old and on its way out so I need to have the cable to get a stable cold idle. The parts were already there from the 616, all I had to do was put that gold color spring in my linkage and the bracket on the valve cover.

[quote=ForcedInduction;1626764]hum, it was sent out on the 20th.


Lance;

I meant I never received a reply from the person listing the car on Craigslist. I did receive yours and thanks for the offer.

John

That knob is handy especially on my tired (270000mile ) 240D. The one on my 1978 300D is broken and I plan to fix it in the near future. From what I've read (can't find the link but I'll post when I do), you need to push the instrument cluster out and it should be accessible. Take off the lower dash panel and push the cluster from behind. Don't kink/break the oil pressure line or you'll have a mess. Some say you have to unscrew the speedometer cable before doing this. The linkage at the knob is where the problem lies and sounds like an easy fix once you get there. Let us know what happens.

Kevin
1979 240D manual
1978 300D auto

i seldom use the knob on my 240D i have it set right in the middle and the car runs perfect

Thanks for the replies. I have not been able to look at my 240D till yesterday. I took the nut off the the idle speed control at the dash, turned it with pliers and noted it did not move the throttle lever. I then noted that the cable close to the knob was kinked into an S shape. I bent it back straight and now it seems to be working ok. Increasing the idle to 800 - 850 rpm makes for a smoother idle.
At 700 rpm, the idle is noticeably rougher. I wonder if the rougher idle has to do with the uneven compression numbers (I'll post them later). I still have to check the valve adjustment but they should be ok per the maintenance logs of the po which indicated they were done less than 2K miles ago.

Who carries the idle speed plastic knob other than the dealer?