Quote: What will that show other than that the water moves.. You already know you don't have a problem at idle.
If for some reason the cylinder pressure is getting into the cooling system you might see some bubbles coming up with the car warmed up and radiator cap off. I have seen this type of issue cause a runaway temp condition that would only happen if the car was driven on the highway and then had to sit in traffic, i.e. at the end of an exit on Hwy 75 at 5pm.
