I forgot the obvious, but thankfully, 420SEL pointed it out. In an engine with aluminum components, you should be changing your antifreeze every two years. My 300E did not have this attention and I suffered severe corrosion problems. The corrosion actually made it into the combustion chamber. I had changed the antifreeze, but the previous owner had not.
I now have an increased awareness of the importance of this commonly overlooked preventive maintenance step.