124 cooling system does not pressurize
 

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?

Mine had an undetectable crack in the nipple that exits the top of the radiator. It's right above the top radiator hose and connects to the small rubber hose that leads to the expansion tank. I had no leaks and no pressure but when I inspected carefully it was damp around the fitting. The new part just pushes into the radiator and seals with an 0-ring. This o-ring may also be the problem.
Hope this helps.

The official MB coolant and/or Zerex G-05 alternative leave a white trail of dried white residue wherever they leak. Don't know if it's done on purpose, but helps tremendously in finding leaks. Perhaps it's time to take a look around with a good light.

- JimY

make sure that the hose from nipple at top of radiator is going to the correct port on the bottle and not to the overflow connection right below the radiator cap.i have seen this mistake made several times.

Good point. However, I'm not losing any coolant. Ironically, I do have to add a bit to the other car from time to time. But not this one.