[trularin]

It is possible you have a high t-Stat and what you are seeing is the transistion from not allowing the water to flow to the opening of the t-Stat.

You start the engine, it runs fine and the temp climbs to say 210. Then it lingers for a mile or so, then it shoots to 250 until the water from the radiator is allowed to circulate through the engine.

I have had this proble on three vehicles. One still has it. The other two got new radiators and the problem went away.

You can check your system by pressurizing it to 15 pounds and let it stand over night. If it is still at 15 pounds ( more or less ) then there is no open leak. Do this when the engine is cold.

Blow into the cooling system is really noticable if you have a puke tank. Basically there would be no way for air to enter the system ( provided you allow all the air to escape when sealed ). If you drive it and the puke tank maintains it's level and air gets into the system, there is the possibility.

Hope this helps!