I wanted my 440 engine to run a a little warmer. It had a 160 degree high flow thermostat and so changed it to a 195 Stant Superstat. 15 minutes of warming up and then started cruising at about 35 mph. As soon as I stomped on it the temperature gauge began to spike and I saw steam starting to come out from under the hood. So I quickly pulled over. The coolant was not steaming from the radiator - it was blasting out the stat housing gasket area. The radiator was barely warm - I could rest my hand on it. So I thought it was a stuck thermostat. (Yes, I did install the stat in the correct direction.) I then bought a new Milodon 180 degree high flow stat. Boiled it and it opened right at 180, give or take a degree. Installed that. Drove around all day around town and on country roads, and a short highway cruise of about 70 mph. Thought I had it fixed. Then, right at the end of my cruise, I decided to stomp on it from about 15 mph and took it up to 75 mph. Here we go again. The temp spiked, I saw the steam, pulled over, coolant blasting through the stat housing gasket again. The radiator was hot this time, but not as if it was overheated - no steam coming out from it.

I know you might be thinking, just put the 160 back in. The problem is the engine always ran too rough, especially on 50 - 60 degree days, as if it never got warm. It ran tremendously better with that 180 stat, but now I have this overheating problem.

Never had an overheating problem and the engine has barely 2500 miles on it. Griffin aluminum radiator, aluminum hv water pump, shrouded, 7 blade fan, no AC, etc. I didn't make any other changes, just the stat. Any ideas?