W126 560 What is this big thing on the engine?
 

Looks like a coolant drain.

Timing chain tensioner.

Really? I never would have thought. It almost looked like it was something added by a PO.

And at 150K I hope your chain guides have been replaced.

Dan

Thanks to the forum, it was actually one of the first things I checked into. I inspected them and they looked newer. They were only a slight shade of yellow. But now that I am getting more and more comfortable working on the engine I will probably replace them for peace of mind.