Truck Repair: boiling coolant, temp guage, radiator cap

Question
I have a 91 explorer 2 wd 6cyl. When driving for longer periods of time my temp  guage will read just above half way but when shut off the coolant will boil over into the overflow.My oil doesn't have  coolant in it and I am at a loss. I have had the rad flushed what   else could it be?

Answer
Hi Patrick,
Your coolant is doing just what it is supposed to do.
When the engine is running, the fan is running, and the water pump is circulating the coolant through the radiator, where it is cooled....to normal operating temp, which is what you are saying.
Then when you shut it off, it quits circulating and cooling, and the coolant sits there and absorbs heat, and expands. Expansion builds pressure, and it reaches the pressure of the radiator cap, and releases into the recovery tank.
later, when it all cools down, weather from sitting, or running again, the coolant in the engine contracts, and draws coolant from the recovery tank to make up for the contraction.

All sounds normal, but if it concerns you, you might have your radiator cap tested, or replace it with a new one.
Also, check that little hose between the radiator and the recovery tank, to make sure it is in good shape, and not allowing any air to be drawn into the system.

Van