I had the exact same problem on my 94 E320 wagon. Turned out to be a stuck thermostat. You can confirm a stuck thermostat by feeling upper and lower radiator hoses during warm up after a cold start. If the lower hose remains relatively cool after 10 or 15 minutes, and the upper hose is warm or hot close to the thermostat housing and cooler closer to the radiator (or has cold spots on the under side) chances are good your thermostat is stuck closed. Replacing it is very easy--takes no more than 10 minutes. You can put it in a pan of boiling water on the stove to further confirm if it is indeed stuck. Be careful when you buy a replacement thermostat--you want one with a design opening temperature of 87 degrees C. (Many parts places will sell you the wrong one.....) Don't forget to drain some coolant before removing the old thermstat otherwise you'll end up with a mess on your garage floor.