I cracked a block. I will keep my fingers crossed for you that you NEVER have to go through what I did. Details here: http://www.wakeworld.com/MB/Discus/messages/559019/579914.html?1212425931
The folks in the know told me that if the raw water intake is somehow starved, you can get false readings on the temp guage...225 if it's just air in the water jacket is possibly engine destroying temps.
One thing I did learn from that was to NOT try and crank the engine over if the plugs are still in. The experts tell me that the starter motor has enough torque to bend a rod if there is water in the cylinders. So if you are going to do what Blair suggest, pull ALL the plugs.
I've got my fingers crossed for it being something simple!