'81 300SD Why is my engine purring now?
 

For the last few days the diesel engine has settled down and just purrs at idle. Generally is tends to have a moderate shake; not quite as bad as an old '60's muscle car with a high lift cam but something like that. A few years ago I had my indy adjust the valves and increase the idle a bit and that helped a lot, but before that I would sit at a red light with a decent shake when, suddenly the engine would just calm down like something had clicked on to tell the engine to smoothe out. But mostly it just shook.

So the increased idle has helped but what could have changed suddenly to make it smoothe out so much at idle? I notice that it does idle smoother in colder weather and shakes more in hot weather. It has been pretty cold in Atlanta the last few days - say 24 degrees - but today is in the mid-40's. Could the colder weather make such a difference? Or is there some non-computer type gadget under the hood that is only working now and again to make it idle smoother?

Pretty happy now with the way it idles; just wish I knew how to make it last. 255K miles on the engine.

The valves should be adjsuted every 15,000 miles. Changing out engine mounts and shocks as well as the tranny mount helps, as does clear air and fuel filters, and running good quality fuel.

Well, I have new shocks, engine mounts and filters and recently (in miles anyway) adjusted valves - but none of this was done in the last few months. So what good happened this week to make it run better?

MB Magic?

My car started going faster off line yesterday
for no reason I could figure.

air bubble or unstuck injector.

You did not state anything about a hot/cold engine. Engines shake more noticeably until at operating temp.

Perhaps you notice that shakes as the car is warming up but then notice it smoothing out when it reaches temp?

There is a fuse in my '80 240D for "Idle Stabilization" Im pretty sure it doesn't have it but the fuse box cover proves that it exists on some of these cars. See what you have and pull the fuse to see what happens.

MB are like cats -
 

-- they purr when they know they are loved :D.

He actually stated that it shakes more at higher temperature. So does mine. I was about to change the engine shocks on the theory that as they heat up, the internal shock valves lose damping ability - but since your's are relatively new my theory become questionable. Driving around town, mine is pretty smooth. Coming into town after 60 miles or so on the highway and I've got a lot of shake. Maybe hot engine shocks is not the reason?

He stated higher temperature WEATHER, not operating temp.

Huh, I never noticed that, but my 85 has "Idle Stabilization" listed under #8 fuse, I have no idea what thats all about:confused::confused:

Nope. There is no such thing as "idle stabilization" on a 240D or 300D/300SD....its all mechanical. On a 240, idle stabilization is that knob on the dash that mechanically raises the idle. :D :D

Only the 1986 300SDL and onward had an electronic idle control.

Your car probably has an incorrect fuse cover from a gasser W123 or something.

Could it be the more dense air with colder environmental temps allowing for more efficent burn of the fuel? (AKA Natural Intercooler)

If so, Alda adjustment needed orr? Slight change in IP timing is needed orrr?

Guys - my diesel has always run smooth under power, just shakes at red lights. But not for the last few days. Smooth as a baby's buttocks. Why - I have no idea. I like the idea of an idle stabilization fuse; with the answer had been that simple all these years.

It will get hot in Atlanta again and I will get the shakes again; sure as anything.

Observe exact same in the SD during summer. Now that it's cold out it doesn't appear to be doing it, coming to think of it. Any more theories?

Shock theory is the only one I have. From a simple physics perspective, the action of a shock has to be better cold.