Thermostat question

pontiacgp

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Bought a 1976 Grand Prix about a month ago and when running highway speed the temp is about 160 to 170 degrees. The car came from factory with 195 degree thermostat but I'm sure the thermostat has been changed at some point by previous owner and I am asuming it was a 160 degree thermostat. My question is that bad for the car in any way? My second question is after running highway speeds at 160 to 170 degrees, when I get off the highway and come into stop and go traffic it will heat up to 220 degrees and drop back down to 160-170 degrees when I start moving at highway speeds again. Any ideas what would cause this?
 

melsg5

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This vehicle has a mechanical fan, it has not been changed to electric?
When the engine is completely cold and engine is off how easily does the fan spin when turned by hand?
The fan shroud has not been removed?
Is the car driven in weather below 40?
 
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pontiacgp

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I will check the fan in the morning and reply back. Fan shroud has not been removed to my knowledge. and the weather today while driving was around 85. The fan is stock I'm sure. The whole car is original except repaint in the early 90's
 
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pontiacgp

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Just checked fan and it turns by hand but not totally free has slight resistance.
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melsg5

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It should have resistance, should barely turn 1 or 2 revolutions.
Is the coolant level correct when cold?
Is the coolant a clear green color?
Do you see anything blocking the front of the radiator, leaves, etc.?
 

pontiacgp

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Nothing blocking radiator. The fluid in reservoir looks good but I will look in radiator in the morning. Took it for a ride today and it seemed better so maybe this car just needs to be run. It has only had a couple hundred miles put on it in the last 18 years. I reply back tomorrow after checking radiator. Thank you for replying to my question , will report back radiator fluid condition.
 

pontiacgp

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Checked the level of fluid in radiator when engine was cold and radiator is full and reservoir is to the full mark also. One thing I did notice is the radiator has some small brown flakes floating on top of the fluid and the radiator cap had a rusty colored stuff I wiped off of cap. What would you do if it was your car? Like I said in previous reply it has only been driven a couple hundred miles in the last 18 years.
 

melsg5

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I'm just concerned why you see 220 readings. If you only do local driving how hot does it get?
Do you think the coolant has ever been changed?
Is the radiator cap marked either 15 or 16 Lbs.?
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pontiacgp

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It only hit 220 once so far and once I started moving again the temp came right back down. It has the original 15Lb cap. I really not sure if it has ever been changed, looks pretty good except for the brownish specs floating around. What do you think about draining Radiator and refill with fresh fluid?
 

melsg5

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It wouldn't hurt if you dont know the maintenance history.
If you buy the coolant that needs an equal amount of water, make sure it is distilled water.
If you feel really ambitious replace the thermostat with a 180 for peace of mind.
Test the new thermostat in a pot of water slowly brought to boil.
It should be fully open before it starts to boil.
You shouldn't have that variation from 170 to 220, maybe sticking open and at least once closed.
 
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