[JSH]

Originally Posted by Gunkk

Syphon/Suction/vacuum as the coolant cools down after the car shuts off. It's supposed to suck the expansion tank fluid (that expanded when it was hot) back up into the top tank. This is how all modern expansion tanks work. It's just counterintuitive with our cars because of the odd arrangement of our multi-tank setup.

I suspect yours isn't working properly because there is a small air leak in the tank that is breaking the syphon and preventing the suction mechanism from occurring, and thus the bottom tank fluid never gets pulled back into the top tank.

Hence why I think replacing the tank assembly will solve your problem.

Are the OEM tanks as good as any aftermarket?

[Gunkk]

No idea.

[JSH]

I bought a Moroso coolant reservoir, but I'm not convinced that's the problem.

[JSH]

I'm embarrassed to say that I just realized that the stock reservoir tank cap is rated at 20 psi, and it's 5 years old. I ordered a Moroso 23-25# cap