I would suspect that there is a leak in your cooling system.

In order to investigate further, you may want to check the front carpet, under your floor mats, if you have them, and see if your carpet is moist or wet.

Also, do you smell the sweet smell that coolant usually gives off.

If so, you most likely have a coolant leak.

P.S. Have you checked your coolant level?? Is it low?

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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 355,705 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}