1988 240DL N/A with about 300k miles.

No problems, except for the fact that I just noticed white smoke (confirmed not black or blue several times and with independent observer) coming out of the 1988 240's tail pipe at idle and increases with acceleration only under load, even after the engine is warm and driven across town. I haven't driven it in a few days, but noticed this when I got back home and it persists. When driving, I don't see any smoke (I double-checked after running all of the tests below).

I think that the catalytic converter is dead from running too rich a few months back, because pre- and post-cat pipe temps are similar. Might that explain the white smoke?

I just checked the following today. Block test was negative. There's
never been overheating and the temperature gauge is a bit cool if
anything today. The coolant level in the overflow tank is still full
and there are no bubbles in coolant. Last year, I changed all hoses,
coolant, and the thermostat. It idles smoothly and drives okay. The
oil is clear; a cigarette lighter to the dipstick did not reveal
bubbles or flame. Oil level was okay the other day.

I removed all spark plugs; the combustion chamber was black. I *think*
the spark plug tips were off-whitish, color easily removed with gloves.

Engine warm WOT compression testing: 152, 151, 148, 167, avg 155 (avg
was 150 on 9/5/19, so basically unchanged).

The other day, vacuum pressure was 20.2" Hg (before I noticed this problem).

After running all these tests, I test drove it. It idled as well or better than it ever has, had good acceleration without smoke visible in the rear view mirror despite flooring the gas pedal.

In short, except for the white smoke, everything is okay. Why is it
still smoking?

Also, what would be the next step?

Thanks