On Sat, 09 Jul 2011 23:37:12 -0400, Wayne B
wrote:

The second is a temperature sensor/alarm at the exhaust
elbow. Either or both of these give you enough warning time to shut
down before damage occurs. Many modern engines have both.

I have a glycol cooled engine, and and elbow sensor would warn long
before the coolant began to boil at 250 plus.