I have an STI engine with stock swirl pot and Wayne hack for engine manifold to pot hose; an overflow tank next to the pot.
Normally it all works well around town on hot days. Overflow might fill 1/4 and return to system when cool. Fans come on about 205deg.

At the track, after 2-3 sessions, it fills the overflow tank and starts leaking out. I usually smell it, look down and see a big temp spike, also note higher oil temp. Look a bit later, all looks normal. After the session, the overflow tank is full.

Not sure what’s happening. I replaced the swirl pot cap in case it is weak, no change. That amount of overflow can’t be just thermal expansion of the coolant. Is a localized flash to steam somewhere causing the cap to release? Is the cap releasing, allow brief low pressure and a steam burp?

I feel like it is well filled with no air pocket cuz the manifold hose and filling slowly with a bucket deal. Stock system calls for a 16psi cap on the swirl pot and 20psi on the radiator. Its tough to believe the pump or thermostat are an issue.

I could try the 20psi cap on the swirl pot? For the moment, I’m relying on the dash temp gage. A water temp gage is in the works. I’m wondering if the ECU drives the gage in non-linear manner. I know my BMW’s were notorious for holding the gage at Normal within a fairly wide range and jumping up to the red when really hot.

I’m planning to add an oil cooler just to lower the heat load on the system.

Any thoughts relative to tracked cars would be appreciated.
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