I found a vacuum line disconnected under the hood. It is attached to some kind of a switch that has three vacuum lines and two electrical wires going to it.

It is located on the firewall between the engine compartment and the area where the air filter is housed. It is actually attached to the engine compartment side and I think one of the three lines also goes to the EGR.

What is this thing and what else does it control?

I also found another vac line disconnected that ran along the fender near the air filter housing.

After connecting these two lines, the engine seems to be running smoother and shifts are much smoother. The brakes also feel more luxury-car-like than before. Very nice...

My question is that while the shifts are smoother now, they are still a bit abrupt. The line that was disconnected is showing significant wear on the exterior, probably because it is exposed the the heat of the engine compartment right next to the radiator.

How can I adjust this assuming this line is not leaking any vacuum?

I am going to replace the line tomorrow to see if that helps.

Also, this might explain why my headrests wouldn't work before. I had to hook up the Craftsman copy of a MightyVac to get them down before.

I will have to reconnect them again to see if this was the problem.

Man, I love these cars.

While I did enjoy all four of my W123s, these cars are a serious step up in comfort and luxury.

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1993 W124 300D
-297K on the clock as I type this.