Zibby, I have had exactly that problem, my engine temperature did not go over 70 even after 20 miles of driving. At first I was (wrongly) happy about it because I thought it should be a good thing when the engine is not getting hot, since the purpose of the whole cooling system is to keep engine not getting hot.

After I replaced the thermostat the temperature climbs now to 87C just after 2 miles and remains stable there.

That was worth 15 bucks of part.

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE