Purge line discussion

[winterset]

We often discuss clearing the purge line when members inquire of an overheat discussion. I always felt the purge line was incorporated into the N* as insurance. What is it purging? Air? I would imagine that it would be helpful during a coolant change, but wouldn't a bleeder screw have worked also? I am starting to believe small air bubbles escaping from the head gasket is normal, and this line removes the air from the cooling system to keep the water pump from getting air bound.

I would bet that the relief line on the side tank cap releases air as a normal operation. Then the coolant is sucked back in from the bottom return line replacing the coolant displaced by head gasket air bubbles in the block.

I am VERY open to comments on my thought.

[Cadillac Jim]

There is always a tiny bit of steam in any engine when running hot and loaded. It forms in the combustion chamber area. The thermostat is never static, but flutters a bit. If a bit of steam accumulates around it when it is closed, water doesn't touch the heat-sensitive element, causing a flash overheat. And then a lot more steam is generated, and pushback on the outlet radiator hose propagates to the surge tank, and you have the traditional loss of coolant.

Every car's cooling system has a bubble purge path. In older cars it was often just the heater hoses and heater core, or if the heater valve closed 100% then or a path through the base of the carburetor or throttle body. Some have it internal to the block. Occasionally it is just a hole about 1 mm wide in the thermostat valve, which isn't enough for most engines.

[Göran W]

Winterset!

You may be part right but...... If you get ANY leak in the system it will suck in air instead of water. That soon becomes hazardous because air expands more than water and press the coolant out while only suck in more air. Very soon gets catastrophic and adding coolant will not help!