I have a 1977 Grand Prix ... I am not a novice mechanic but this problem has me buffaloed I have put a OEM 3 core radiator, water pump a 180 degree thermostat, new radiator hoses and new clutch fan also. The problem is when the car idles with a thermostat it runs hot very quickly. When the car is idling without a thermostat in it, it hits about 165 to 180 and goes no higher. No matter how long it idles. I've clean the radiator inside and out, doesn't seem to be stopped up. It start circulating fine when the temp hit 180 with the thermostat in it but keeps on going up to boilng. I don't have any white smoke coming out of the tail pipes, no oil on the dipstick or under the radiator cap. New radiator cap, changed the plugs, no water on them either. So what can cause a car to run fine without the thermostat and overheat with one. Also have checked the compression and all cylinder are compressing to 120. Any information would be appreciated ...MikeT.