constant overheating

[Rems66Pont]

Has anybody out there experienced constant overheating on a 326 motor? I have tried everything from aluminum water pump and radiator, and installing a shroud. I know Pontiac motors are known for overheating but mine overheats one minute after start up. I changed the thermostat twice, bled the air out of the system, and still it overheats. the thing is the radiator shows a temp of 90 degrees and the thermostat housing shows 140, not even enough to open the thermostat. I wanted to keep my car original but I am about to haul the motor off to the scrap heap. Any suggestions would be greatly appreciated

 

[melsg5]

it's not clear how you are determining the engine is overheating if you have a radiator temperature of 90 and a thermostat housing temperature of 140. I assume you used an IR gun to get those two readings?
What symptom are you experiencing that makes you think it is overheating if the temperatures are nowhere near overheating?

 

[Rems66Pont]

The coolant is steaming and running out of the overflow. I find it strange myself, that the engine and hoses are getting very hot but the radiator seems cooler. I did use an IR temp reader. New water pump was installed at time of rebuild so I'm guessing it should be working properly. That is what baffles me, a friend removed the radiator cap and was checking with his finger to see if the coolant was getting warm and the coolant came gushing out of the radiator and was still cool. The reason I am calling it overheating is as mentioned before, the engine and hoses are getting very hot within 5 minutes of running

 

[Rems66Pont]

I say steaming out of the radiator but bubbling out more accurate. Two weeks ago a friend removed the radiator cap and held his finger inside the radiator to determine if the coolant was getting hot, the coolant bubbled out but was not even warm. I describe it as overheating because the engine and hoses seem to get pretty hot even after a couple of minutes.

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[melsg5]

Are you sure you are using the correct radiator cap?
Did you use a cooling system pressure tester to see if the cap is defective.
The thermostat you installed has to be 160 or higher thus it will not even begin to open until it reaches the rated temperature of the thermostat. As long as it is closed the engine will build up heat and the radiator will stay cold. So far the only thing I suspect is a bad or wrong radiator cap.

 

[Rems66Pont]

I will try a new radiator cap, it is a very good possibility that it is the wrong one. The thermostat I have is a 195 which it calls for. I appreciate the help and will start with the cheapest fix first (the cap)

 

[melsg5]

Recommendation on the thermostat, the only reason 195 thermostats came into the market was to meet emission requirements. In 1966 the correct thermostat would be a 180 and if you are in the really hot section of Texas I would go as low as a 160.