I've got a '79 Chevy C-10 pickup with a 305 under the hood, and every time I put a thermostat in it, it overheats. I've tried 3 or 4 thermostats, and I know I'm putting them in the right way, but it'll run for a few minutes, get hot, and stay hot. Eventually it'll overheat and stay there until I shut the engine off, then it cools down and will run cool for a while until it gets hot again. It seems to mostly do it at speeds over 30 mph, and the upper radiator hose collapses when I hit the gas. I've tried flushing the cooling system to no effect, and have tried numerous thermostats. Without a thermostat it runs cold as hell. I'd leave it alone, but without a thermostat it runs like crap and burns a lot more gas. I'm pretty sure its not the headgasket as my oil and coolant look fine, and I've got no smoke coming out of the tailpipe. Wanna kill these ads? We can help!