Update …
I’ve done a perfect bleed, just burping it and verified that the gurgle in the heater core is 99% gone. I did this using the spill-free-funnel and the car on a incline, modulating the rpm a bit and idling. That takes a good 25 minutes to do.
Then while mellow driving around it seems to hold the state and no new bubbles are introduced.
To get the problem to recur, I drive up a hill nearby that has approximately a 500 feet vertical rise over 1 km. The bubbles don’t appear to manifest at the top of the hill, but in fact after I get down the hill using engine braking (1st and 2nd gear, often sustained 4000 to 4500 rpm).
So … even after a perfect bleed. air is getting back into the system after either 1) a hard drive or 2) a hard engine brake after a hard drive.
To check if combustion gases, I bought an OEM test kit tested twice, for even longer than required, and both tests came back negative that it is not combustion gases.
So this really has me stumped.
1) No loss of coolant. Either reservoir goes a little higher (from air in system displacing coolant) or it goes back to normal, after air is burped and system re-capped.
2) No leaks. Vacuum test for 4 hours, no leaks. Pressure test 4 hours, no leaks.
So … something is either sucking the air into the system (on cool down when engine braking at high rpm??) or under high load.
The mystery continues.