I don't understand "Open Non-Pressurized" system ... if you pull negative G's coolant can spill out somewhere??
Regardless, if you have coolant O-ring problems you will likely see coolant in your exhaust ports if your remove the pipes and roll engine over by hand.
Jeff

Over the last few years I've noticed more and more coolant (Evans) in the overflow tank.

This is an open non-pressurized system -- a header tank on top of the firewall -- bottom outlet goes to the pump inlet, and near top to the thermostat (cap) filler neck.

I though the problem was that there was a small leak in the system somewhere so coolant was not being sucked back into the header tank and engine as the engine cooled down.

Now I'm beginning to think that it could be blown (or weak) o-rings allowing compression/exhaust gasses to escape into the cooling passages, forcing out the coolant.

In a normal pressurized system, one should be able to look at the coolant pressure gauge (I don't have that -- no sensor) and see pressure increase as RPM is increased if there were blowby, I guess.

But how do I do it in a non-pressurized system? Add a pressure gauge and temporarily block the hose to the overflow tank and run the engine for a few seconds?

I'd rather not do an engine rebuild if I don't have to. Any other ideas (than coolant o-ring) of what could cause coolant to accumulate in the overflow tank?

One more data point: on flight before last the water temp (sensor mounted near top of pump housing -- filler neck) dropped to 60F (same as with disconnected sensor), went back to land, and during and after landing temp showed 185F. I figured it must have been a bad connector, but was worried about lost coolant. Later I pulled the cowling and found about 1/3 more in overflow tank that what I'd seen last time the cowling was off.

Finn