I have a similar problem. It started when I replaced the stock air separation tank with an aluminum one. It isn't coolant seals, I have great compression and no bubbles or residue in my coolant. What I notice is that I need to fill the coolant at the cap every other week. Meanwhile my overflow is totally full. Bottomline, it is not sucking back to the radiator when it cools down. Must be a sucking air from a bad hose lack of seal on the aluminum Air separation tank or coolant fill cap. I have replaced the fill cap but no change. I guess I need to pressure test the system.

 

mecman,

Since what you changed just before the problem started was the AST, I would suspect a bad hose or connection between the AST and the overflow tank, or a bad seal of the AST cap outer seal to the AST, either of which would result in coolant not being drawn back to the engine.

As you said, ether of these would cause what you describe.

A pressure test may not help with the diagnosis, since, under pressure, the system is working as it should.