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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.
 

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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.
What does your fan shroud look like? What coolant/water mix are you running? The thermostat is there to make the car warm up quickly.
 

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What does your fan shroud look like? What coolant/water mix are you running? The thermostat is there to make the car warm up quickly.
It does that but it hit s 230 and up in about 3 to 5 min according to the gauge. The fan and shroud are both originals... The engine components are all about as original as you can get..Ive heard of burping one but have never done that on any car I have owned and that's been a bunch of them...Anything to the burping theory? I've never had a engine problem like this and it does have me scratching my head a bunch....Thanks for your time and reply.
 

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It does that but it hit s 230 and up in about 3 to 5 min according to the gauge. The fan and shroud are both originals... The engine components are all about as original as you can get..Ive heard of burping one but have never done that on any car I have owned and that's been a bunch of them...Anything to the burping theory? I've never had a engine problem like this and it does have me scratching my head a bunch....Thanks for your time and reply.
Assuming your water pump is good, an air pocket might explain what you have going on. Does it stay cool while driving?
 

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Well may sound strange but I have driven it yet on the road was afraid to try it out with it acting like it does... It runs and cranks (knock on wood...LOL ) like a dream except for the problem I m having.. Don't want o make matter worst by running it hot. I guess I need to bite the bullet and see what happened but first I will try burping the engine before I do. Thanks again for your help.
 

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Aside from the mentioned air pocket, I suspect the new thermostat is defective. The only way I know to confirm this is to put it in a pot of water with a thermometer and heat it to see when it opens.
 

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Is the clutch on the fan engaging when the temp hits around 190? Make sure its not just free wheeling. It needs to engage and pull air.
 

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I'll follow whatnext's lead and go with a faulty thermostat.
Or is it possible a hotter t-stat got in the 180 box/packaging by mistake....been known to happen....
My first car was a 76 Grand Prix SJ with a 400 in it, great car.
 

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Yes BASG8 they are. I had a 76 also with a 350. heck of a car it was. I checked the thermostat in a pan of water and it opened around 180 also drilled 2 .. 1/8th inch holes in the side of it and it did seem to help. I ran it around the block about 7 times it hit 185 and held for awhile and then started climbing to 200 . 210 and 220, 230ish ..I ran it about 20 minutes and then came back home when it hit 220-230..Pulled it in and checked both top and bottom hose both were hot No overflow, no steam. I'm hoping its a faulty reading on the temp gauge but I am getting the original gauge fixed and goin to reinstall it with a new sensor. Thanks for yalls help and if you fellas have any more suggestion I'm open for them... never to old to learn something new.
 

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If there is air in the system it will overheat.
 
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