124 cooling system does not pressurize

[brewtoo]

On my 1990 W124, the cooling system does not pressurize. On my 1991, it does. I have swapped the cap and it made no difference.

It loses no coolant and does not overheat. But without fail, if you open the hood and check after you drive it, you can actually remove the cap as if the engine was cold. You wouldn't dare do this with the 1991 without a few rags over the cap.

Wonder what's going on?