Does this sound like the rack damper bolt?
 

Do these engine shake symptoms sound right for the rack damper?

- Generally no shake or very slight shake at full operating temp.
- No shake when started from dead cold (car has sat overnight.)
- If I warm it up, shut it down for a couple hours, then restart, it shakes like a paint mixer at idle until it fully warms up.

It is behaving like an Air Leak to me.

I had a more mild but similar situation. It turned our my Fuel Inlet Hoses were hard an inelastic. Even though the Clamps were tight the Hoses were not sealing on the Plastic Filter. The little bit of roughness I got from that only showed up after everything under the hood was hot.
(Hand Primer?)
When it is running rough like that try pumping fast on the Hand Primer and see if it goes away while you pump.

No, not the rack damper bolt. You would have the problem any time the motor was at full operating temperature, were it that.

Diesel911 has raised one possibility, another would be that some component of the injection pump is out of spec and after you shut the car down, the resulting "heat soak" (absorption of heat from the motor by the pump) is aggravating that.

My '79 SD had a similar issue and various mechanics either indicated the governor inside the pump, or something to do with one of the pump plungers.

The problem was 'solved' when I installed used motor after the timing chain snapped on the original motor :eek:

Well, that's not reassuring.

WHOOPS let me try that again
 

Sorry, I did not mean to connect the two. Rephrase: I simply lived with the problem for six years until, one fine morning, the timing chain snapped.

The issue was that some genius used 'McChain' on the car when the motor was rebuilt. Had nothing to do with how the car ran.

Ah, I see. Makes sense.

"Just live with it" is certainly an option I'm considering, especially since putting a little pressure on the accelerator stops the shaking. Will also try the primer pump suggestion to see if maybe it's air in the lines.

The car definitely needs the left engine shock replaced and probably a valve adjustment, too, so it's possible those will help some. And your post is a good reminder that I need to check chain stretch when I have the valve cover off. ;)

What's the tach showing at idle?

Is it idling slower, when it's hot (and running rough)?

I've been thinking about my first reply to you, and I'm reconsidering it.

Reason: As I understand it, the purpose of the updated rack damper bolt (the gold one) is to correct for natural wear in the pump as the car ages. It's just a touch longer, which restricts the movement of the pump 'rack' when the engine is at operating temps.

The theory is that you always have to adjust idle to find a 'sweet spot' based on adjustment of both the idle screw and the rack damper (and some would include the ALDA as well; IMHO that's overkill).

Each engine is different as it ages. Some are happy to idle way down at 500-600 rpms, some want to be at 700 or more.

Perhaps you may want to consider getting the updated bolt, and messing with your adjustment. It's not something I recommend lightly, as it can drive you nucking futz, getting the sweet spot nailed down. BTDT on three cars, two weren't bad but the third nearly drove me insane. (And as soon as I got it just right, some ditz on a mobile wrote it off at an intersection.)

But before you go that route, you do need to ensure everything else is up to scratch -- no air leaks, dirty filters, etc.

No tach, just a clock, unfortunately. By ear I don't think it's running at a significantly different speed.

Your car IS a turbo, right? Just checking.

Also, pls note I edited my post while you were replying to its original form :o

Yes, it's a turbo, but the '81 wagons didn't get tachs. :) It's a weird year. The instrument cluster is unique, the '81 wagon was the only W123 to get an electronic speedometer IIRC.

Thanks for pointing out the update. I think I will give the rack damper a shot eventually but there's other stuff I should deal with first. It still has the original silver rack damper bolt, though.

Right now it's at the mechanic. I took it in for a noisy wheel bearing and that's gradually turned into a bigger job as they took it apart.