Hey everyone!

I have a 1995 e420 and am having some issues with the engine running cool. I'm in MN so the temperatures are now hovering around zero and my engine no longer reaches 80. It comes up to temperature if left at idle but as soon as I'm driving the temperature drops to 60 and stays there until the car comes to a stop again. This tells me the coolant is running in open loop pointing to the thermostat being bad. This weekend I replaced the thermostat with an OEM Thermo from the dealer. Same problem. My question is, could there be ANY other reason other than the thermo as to why the car is running so cool? My guess is that I might have gotten a faulty part but I don't want to assume anything until I've at least reached out to the community and gotten a little input.

Thanks in advance for your help!!

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL