1980 240D vibrates excessively at idle

I drove my Dad's MB recently, and noticed excessive vibration when idling. Car has automatic transmission, with "drive" loading engine at idle, thus
lowing RPMs below that observed with transmission in "neutral.: Vibration (and obvious chassis resonance) is terrible at low RPMs. Could not have been this bad, when car was new. So that makes me suspect motor mounts.
Casual inspection of 2 large motor mounts at sides of engine reveals nothing suspicious. Has anyone solved this problem?
Autozone tells me there are as many as 6 motor mounts.
Vehicle has about 180K miles on it.
Thanks in advance for advice.
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Use the dash mounted idle speed adjustment knob to raise the idle rpms.
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And remove the center stop on the bottom of the oil pan.
#68 here: http://www.detali.ru/cat/oem_mb2.asp?TP=1&F 3123&Ma6%2E912&GAr2%2E117404&GMq6%2E005210%2C214+++++++717%2E400&CT=F&cat"2&SID$&SGR5&SGN
It is adjustable as the two engine mounts sag with age. PITA to remember to do at every service. Easier to discard it.

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I believe I see item 68, which is a right-angular bracket the supports the front of the oil pan. There appears to be a rubber shock isolator there. I doubt I'll remove that now, but thanks for the tip.
I am amazed at how much this engine vibrates. Regardless of engine mounts, I would expect any properly tuned engine to run more smoothly, but I am inexperienced in diesel engines. Can you offer an opinion? Should a 4-cylinder diesel engine vibrate badly?
I suppose anything would vibrate at its resonant frequency. I suppose that even a V-8 gasoline engine should vibrate, if its idle rate were low enough.
The vibration decreases DRAMATICALLY as soon as the engine speed is raised a little bit. There is an obvious resonance phenomenon at work, here.
I suppose, if the shock isolators are dead, then there is nothing to dampen vibration.
I have a CDROM describing some mechanical features of this '80 240D, and quite a number of other models. I notice that a more expensive auto model, the 1985 300D, has two "engine shock absorbers." There is no picture, but I expect these resemble fluid-type shock absorbers common to all automobile suspension systems. This tells me that 4-cylinder diesel engines naturally vibrate, and various shock-absorption methods are employed to dampen the effect on passenger comfort. I have never heard of anything like this on the 4-, 6-, and 8-cylinder gasoline engines I have worked on during my lifetime.
Any comments welcome.
Thanks.

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There are two small knobs on the dash, above the steering wheel. One controls a rheostat for dimming console lights. The other seems to be a dummy. Just below, to the left, is a knob that has "ears," sort of like those on a key. I cannot turn it. Is that the knob that is supposed to control idle speed?
Thanks.

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John B wrote:

The dimmer one also resets the trip meter when you push it in, and the other one sets the clock when you push it in and turn it. ...or so it is on my '79.

Probably. You can look in the engine compartment, it goes through the firewall and arcs in toward the STOP lever, connecting to that linkage just below where STOP is printed. You could fiddle with it on that end, to see if anything is obviously wrong there.
-tom!
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that is the idle control. It only moves ~180 degrees on mine. I do have the orig owners manual I can check if you need any info John B wrote:

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I turned it to and fro, perhaps + and - 90 degrees, but noticed no change in idle. Next time I visit my Dad, I'll open the hood and examine the linkage. It turns reluctantly.
I'll get back here in a week or so, on this. Thanks.

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FYI It may seem strange at first, but the engines in 300D model Mercedes have 5 cylinders.
72 x 5 = 360, so it works.
There is a spring loaded screw on the injector pump of my 300 TDT's engine that I had to replace about ten years ago when the engine was wobbling at idle. I don't remember the name of it right now, but the parts agent at my local dealer knew about it and recommended replacing that screw when I had the problem of excessive vibration when stopped at a light with my car in Drive and my foot on the brake.
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That was a rack damper bolt and new harder spring. It's only on turbos.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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