I'm stumped!!! My 1988 190E, 2.3 is eating coolant. Here's the situation:
Daily the light comes on and the level is low. When the engine is hot there is no pressure in the hoses- I checked the cap, it's bad. We then pressure checked the system and it held pressure for more than five minutes with only a slight pressure loss. We started the car and it ran rough, blowing white cloud for about three minutes down the road.
The next day it was low but started and ran normal - no white cloud. I haven't replaced the cap yet but as long as I keep topping it up it runs normal. My question is if it were a head gasket or cracked head wouldn't the car be blowing white and running rough all the time? Surely the compression of the engine would be way higher than the cooling system?!

Haven't had much head gasket experience with MBs, but I've seen Asian cars do this....blow a cloud of steam one day and not the next. In these cases, the head gasket was getting weak and eventually gave way. Once that happened, the oil looked like a milk shake.

It sounds as though you're losing a significant amount of coolant daily. I'd drain the oil and take a look at it for starters.

Your car won't necessarily blow white smoke or even run rough. I just had the head gasket replaced on my 91 190E 2.3, and it ran great but kept losing coolant. It was just a pin prick size hole, but that was enough under pressure to keep it leaking.

I'm working this same issue right now, from what I've been able to gather and try, I put the overfill hose at the fill opening of the reservoir into an empty bottle. The car only dumps out this hose when the pressure inside the coolant system exceeds the 25lb rating of the radiator cap. This only happens when your driving down the road. Everyone says the head gasket is blown between the piston and the water passage, I'm pulling the head on mine tonight and we'll see. Just had it replaced a month ago, won't be going back there, can't count on people to a better job then you can do yourself. Good luck. Don't wait too long to fix it, water and oil will eventually start mixing, next the crank shaft bearings will grab the crank and start spinning, more work.

These engines have TORQUE TO YIELD HEADBOLTS . . . FOLLOW DIRECTIONS VERY CAREFULLY ! ! !