I have a 82 240d that is only getting about 21mpg and is the most sluggish beast I've ever driven. It runs good, always starts and is reliable, just no power.
Any suggestions on things I can check and attempt that can lead to better performance without a $500 trip to the shop?

I'm thinking things like, diesel fuel additive, swapping out parts, etc?
Anything obvious that I could be missing? I'm not too diesel savvy.

Thanks for any help.

Short of engine and tranny exchange, live with it or trade up to a 300D turbo. 240D's are sluggish. That's why they call the front and rear windows "windscreens".

-Scott missing my old 240D 4-speed

Running diesel purge through the system may help, as would changing the filters at the same time. A commonly neglected item on the older diesels is valve adjustment. If mileage is very high, then injection timing and timing chain stretch may be issues.

As Scott said, the 240's are very sluggish. An upgrade to a nice used 300D turbo may even be cheaper and more rewarding than pursuing timing issues if you don't do the work yourself.

...AND, I am not sure how dramatic a difference it would be in your car, but when I suddenly lose power and smoke alot, it is very often a vacuum leak! Usually easy to locate, just look for a loose connection, though sometimes they are a bit trickier.