rdanz,

The coolant volume is part gas and part liquid. As the engine warms up the metal, liquid and gas respond according to Mother Nature. The gas response is the most noticeable, but the metal grows and the vapor pressure of the liquid increases. This brings about an increase in the internal pressure as long as the system is sealed. And yours is, up to a temperature and pressure limit.

As you leak a little steam, or water vapor to either the overflow tank or the ambient air under the hood, the volume inside the coolant system just changed. This leakage is controlled by the cap, which is really a two way relief valve. When activated internally to relieve internal pressure, steam or hot gas leaks until the volume change has reduced the internal pressure enough to fall be low the cap set pressure (1.4 bar, or 20.58 psi).

When the system cools off, the stuff that was steam inside the coolant system condenses and turns back to liquid, minus the volume that escaped. This involves a volume change on the order of a factor of 2000 as the steam returns to a liquid (steam is on the order of 2000 times the volume of the liquid water that it was before it changed state). Meaning if a liter of steam escapes it might only be a few drops of water (less than a milliliter), but to the inside of the coolant system, it is the equivalent of a liter of volume missing. This creates a vacuum, which the second "way" of the the two way relief feature on the cap is supposed to relieve. For some reason, this feature is ot working on your car and the soft parts of the system get squished by the fact that the internal pressure is now less than atmospheric pressure, meaning your hoses get squashed.

I would tend to agree it is not a real disaster, but it is not the way the system is designed to work, so they should fix it. Ask the guy if the system has a vacuum relief feature in the cap, and if there is an overflow tank. Better yet, lift the hood and look for yourself. You should see a small tube that is connected to the neck of the expansion tank (the tank with what used to be the radiator cap on it) under the cap. The tube should run behind the passenger side wheel well, through a partition, to the overflow tank that is out of view. If you do not have the overflow recovery system the tube will run to the ground below the engine. Show this stuff to the guy and ask him what it is for and how it works. If he/she cannot explain it see the manager, then ask for the district MB service representative.

As noted, MB has test equipment and a procedure for checking the cap. I think it would be less expensive to just buy another as the labor rate for the test is likely to exceed the cost of the cap by an order of magnitude. But the tube going to the overflow tank, or the tank itself could have a problem as noted above, so the cap may not be the culprit.

If your car did not used to run near 100 degrees Celsius, then the fact that it does now is not normal. This acceptance of 100 degrees as a normal condition is not with what my experience is for normal on many MB's for a long time. I would consider 85-87 degrees Celsius normal except under highly stressed conditions, like over 90 degrees Fahrenheit outside temps, and either stop and go traffic or pulling a trailor up a hill. But that is my experience. Go to the dealer and tryout a new car and see what it does on the same route you just drove, then ask the service manager why that car runs under 87 degrees Celsius under the same conditions yours runs closer to 100?

Like I said before, there are relatively few parts that can go wrong, and the prime suspect in my mind is the thermostat. MB coolant systems are more complicated than most and the thermostat is both a temperature regulator and a mixing valve. Either function can fail and disrupt the temperature regulation feature. Another source of trouble is the clutch that makes your belt driven fan turn. I have not seen this personally, but others here report it is a source of trouble. And then there are the electric fans and the sensors that turn them on. These are easier to check (let the car heat up, turn the A/C on and the fans should come on in stages and speeds) and easier to fix.

Good luck, and keep after the issue. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)