My 2.8 with mechanical fuel injection seems to be running rich at idle and it's missing with the pedal all the way to the floor. Tailpipe is black too. Probably running rich at mid-range rpm's too but seems OK in that range. Plugs are black and sooty also.

Any thoughts on where to start diagnosing?

buy a fuel pressure tester for the kjet system and read the instructions. the pressures listed with the kit are close enough to let you know if it's control pressure causing the problem. good luck, chuck.

Thanks.... anyone else have any theories?

The thermosat for the injection pump could be bad. If that's the case the idle should be high and drawing air through the small filter next to it. Or the coolant level could be low, not allowing hot coolant to reach the injection thermo. The cold start squirter could be leaking or it's control circuit holding it open.
Has any work been done to the engine recently, or did this just pop up?

Don't know.... I'm merely the new owner. Previous owner did not drive it so I have know history. Idles normal, but rough. WOT the engine misses from being loaded with fuel.

Here's what I'd do (and I have) with this one. I'd forget about overthinking the thing and do the very basic tune-up things....plugs, cap and rotor. Go from there....that's what I'd do.....I just did this with my first Mercedes Benz last year so I have a little bit of experience, not alot, but a little...good luck

but regardless, that was D-Jetronic. The GrandFather of Bosch Fuel Injection systems...the D is large.......I had the same thing on my very first car, a 1973 VW 412 with a Porsche built flat 4 in it. D-Jetronic is very primitave, I am just sitting here reading about it in my manual....

Actually, that is very sage advice. I'd also add valve adjustment, COMPRESSION TEST, check for vacuum leaks, exhaust leaks, etc. Check cam timing/chain stretch. Ignition timing is implied in your post... include centrifugal and vacuum advance function.

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1987 W201 190D

Am I missing something? Isn't your car equipped with the Bosch 6 plunger mechanical injection system? This is not D or K jet. You would probably get better info if this post were moved to the Vintage section. Anyway, these engines tended to run rich when properly adjusted and do not like stop and go, short trip drives. Do a proper tune up including air and fuel filters but also check the coordination of the throttle linkages---these frequently get misadjusted/bent causing a rich fuel mixture (the injection pump lever is opened too much relative to the air venturi lever). Do not adjust the idle mixture on the injection pump until you know what you are doing. $$$ or personal injury. You definitely need to read up on this before proceeding. Mark

I remember when I was first starting out as a mechanic's apprentice in about '77. We worked on lots of these cars, and vacuum diaphragms of that vintage (really any Bosch vacuum diaphragm of the time) were a common failure. Easy way to check it... stick a vacuum hose on it, and suck on the end. You should be able to move the arm with your own suction. If it doesn't move, the breaker plate may be seized (not too common, as I recall), or the diaphragm is bad. Frequently with a bad diaphragm you can still suck it hard enough to move it. If you plug the end of the hose with your finger or your tongue the capsule should hold it's drawn-in position essentially indefinitely. If the diaphragm is bad it will slowly release over a couple of seconds.

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1987 W201 190D

While you have the cap off, give the rotor a little twist. It should rotate in one direction a bit, and spring back to it's original position. It should not rotate in the opposite direction. If it doesn't want to turn at all, the centrifugal advance is stuck and the car will be a dog.

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1987 W201 190D