I am going through this problem with my 300SE right now. Just replaced the thermostat, and it made no difference. If the car isn't running hot, then your problem is most likely the dreaded monovalve and not a sticking t-stat.

The problem I've seen with some non oem thermostats is the gasket sucks compared to the proper O-ring that's supposed to be in there. I just got the t-stat with the O-ring at the stealership for $27.


J.