Sounds like a good guess. You can try another simple test - when the car is cold, squeeze the upper rad hose - you should be able to hear the drizzel valve jiggle (it's that little hole in the thermostat and it's valve) as you force coolant thru it. If you don't here the sound, that too would inicate that the stat is open.

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Tom Latos
2002 E320
1995 E320 Sold at 288kmiles