The block heater has only a small heating coil, so the entire engine wont heat up enough to register on the coolant temp. gauge. (You have 12 quarts of coolant, plus the entire mass of the engine to heat!). But, even on the coldest sub zero mornings, after my block heater has been on for a few hours, the top of the valve cover feels luke warm to the touch.....

The heating element itself gets very hot.....yes you can hear it cooking! This doesnt take long...plug the thing in and listen....within a minute you should hear the little thing bubbling in there. If not, you need a new one.

They wear out with age....you can get by with a marginal one, but when you put in a new one you will be surprised at how much cooking noise it makes.

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)