I don't understand the coolant bleeder/tank system

[JFP in PA]

Originally Posted by Dale_K

On my American vehicles there is a pressurized expansion tank at the high point of the cooling system. There is no bleeder valve, just a medium size hose entering at the base of the tank and there is no bleeding during refilling. Just keep adding coolant until you get to the right level recognizing some air will migrate to the tank at first and you might have to add a little more coolant.

Therefore I don't understand the reason and function of the Boxster system. Why are there two "caps"? If you never touched the bleeder would any entrapped air move into the pressuized tank? What does the diaphram inside the bleeder do?

I'm considering a coolant flush and replacement on the 2000 S I bought a couple of months ago. I just want to understand the system before I start working on it and have never encountered a separate bleeder except I have seen distinct bleeders on the motor plumbing when the up and down routing of hoses and components left an isolated high spot in the system. My 1990 Thunderbird had a bleeder vent on the motor.

The reason for the bleeder assembly is quite simple: Large sections of the cooling system lie below the coolant levels inside the engine. As such, air can become trapped, particularly in the radiators way up in the front of the car, causing poor cooling, “hot spots”, and associated steam pockets, which lead to further air entrainment and potential engine damage. There are two possible ways to get the air out; the easiest, fastest, and most effective is to pull a vacuum on the system which will immediately purge all traces of the air; but what happens if you do not have a vacuum filling tool, or are stuck out on the side of the road with air in the system? You add a mechanical bleeder as high up in the cooling system as possible, which is where the tank is located. Manually open the bleeder and the system is running at atmospheric pressure and trapped air can vent as the coolant circulates.

Believe it or not, most hot water home heating systems have a similarly functioning device for the exact same reason………..