The head gasget on my 1987 300E suddenly decided to blow. I know of no apparent reason why. Does anyone know if the engine has a steel or an aluminum head?

The OM603 engines like yours has an aluminum head, not cast iron.

Are you sure the head gasket blew? Last year, when it appear that I had blown a head gasket, it turned out to be corrosion between water jacket and combustion chamber was the problem.

You won't know for sure until you pull the head off.

Best of luck,

With due respect to Doc, I'm quite certain that you have an M103 engine in your car, as it's not a diesel automobile. The head of an M103 motor is indeed aluminum.

Do a search-M103 Head Gasket issues have been discussed at length. I suggest that you do a full valve job (if not already done), if the head has to come off anyway.