So I replaced the radiator, water pump, thermostat, and fan in my 66 dart. Filled it up with coolant, started her up ran the heater. Ran hot according to the gauge. I drove it around the block and it cooled way off. So the coolant cycled through. Parked it in my driveway and let her run for 30 minutes. Everything seemed ok. Temp held at half. So I take her for a ride. Soo as I get going she runs real hot all the way to the left. I get to the stop sign and she starts to cool off. Drove on, hot, stop cools. Shows no sign of overheating. What gives?

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66 dart 225

Sounds like you have air in the cooling system.

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Joshua

Find a hill. Park on the hill with the front of the car facing uphill. Make sure the hill is steep enough that the radiator cap fill hole is above the thermostat housing. Run the motor with the radiator cap off until the motor is hot and the coolant is flowing. Replace the radiator cap, drive home, and fill the radiator as necessary until the coolant level is 2-3 inches below the top of the fill hole. You can use a floor jack in place of the hill.

The purpose of this is to make sure all air gets removed from the cooling system.

How can I fix that? And is it really running that hot?

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66 dart 225

Do I run the heater? Thanks.

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66 dart 225

Run the car for like 15 minutes after it warms up, heater running, radiator cap off, idling.

Don't look down the radiator, if it burps sometimes some HOT water can come out!