Is this one of the ones that has the throttle valve in the intake manifold? If so check and see that it is openeing. it could stick and not let enough air in, and of course that would cause black smoke.

Does anyone make a clear cigar hose? Don't know.

I should have put "clear cigar hose" in quotes. That was just to tell you where the clear hose should be placed. Make a temp cigar hose from clear 5/16 vinyl tubing from Home Depot. You probably won't be able to find hose clamps small enough so just use a heavy duty rubber band and wrap it around a number of times then tie a knot.

I remember taking the inlet manifold off my 83 240D when I stripped it for parts and it had a throttle flapper valve. Check that it is not stuck as suggested already.

Thanks for the info. BTW, do you know if I have an EGR valve on a 1983 240D? If so, I have not been able to find it.

Yes, I checked the flap valve too, thanks.

In post #14 above I wrote: "My 83 240D had a egr computer which I modified to drive a tach. Did a DIY on it."

Since it has a EGR computer, that was meant to imply there are other parts to complete an EGR system. Yes, there is a EGR valve plus a bunch of other parts that make up the EGR system. Take a look at the electrical wiring diagram for an 83 240D, that is the best place to look at all the parts. The EGR valve should be on the exhaust manifold staring right at you. Take a look, you can't miss it. If you can't find it, maybe the engine was swapped out for an older one? I remember taking the EGR valve off my 83 240D when it was stripped for parts and cleaned it off nicely to study it. It is not unusual for it to be stuck in a 30 year old car. If you need a good one, I have it.