[Razorback1980]

Here is why a blown head gasket overheats at idle. The reason the gauge reads hot is that the air being pumped into the cooling system is very hot...not sure what it runs on a Porsche engine but probably in excess of 1000 degrees. That tends to make the water get pretty warm, however at high rpms obviously much more air is pumped into the cooling system building pressure much quicker and that opens up the radiator overflow cap and the air pressure escapes. With the hot air gone, water temp goes back down. At low rpms, the opposite happens. It takes longer to build up pressure because there are less rpms so the hot exhaust gases remain in the cooling system for a longer time....therefore the water temperature goes up. Also at idle, there is not an extensive air flow going through the radiator which compounds the problem. Usually at high rpms you are traveling down the highway and the amount of air flowing through the radiator is higher. Fans move some air but come no where close to traveling down the highway at 70. Put your hand out the window at 70 then feel the air flow in front of the radiator to compare the two. Much much faster air flow at 70.

Do the pressure test if you have the equipment and also check out the compression numbers. The two will tell you a lot.