Larry is right. Until recently American thermostats weren't made to fail open. This is of course a much safer way to go, than to have a thermostat fail closed and have your engine overheat to the point of wrecking it. Change your thermostat. It also might be a good time to change your coolant.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired