So my cooling system on my '10 R55 justa has been a bit of a nightmare for me for a while now.

Backstory: Bought it back in May '18. Had to replace the water pump w/ a cast housing (not the composite housing) in Nov '18. To add to this, it seems like the wrong coolant was in the system before the WP replacement. Drove it around for a while, and the thermostat housing started leaking). Replaced the thermostat housing back in Apr/May this year. Still had some overheating issues until I finally broke down and bought a coolant vacuum refill kit. First time I tried to pull a vaccum, I lost the vacuum. Found a small weeping leak at the endcap of the radiator on the driver side. Pulled out the radiator and replaced it back around June and vacuum bled it which held fine, and filled. Been driving it around fine, few thousand miles...until Saturday.

Drove the car to work but used the heater that day since the morning was chilly. I was getting solid heat out of the vents. No issues, until I drove home. Stopped at the bank drive through on the way home (car was fully warmed up), and once I started driving again, temps shot up to about 108-110C. Nursed it home and shut it off and the expansion tank was about 3/4 or more, full. I let the engine cool overnight, uncapped expansion tank, engine off, heater at max and low speed fan, bled all the air out at the bleeder screw, and topped off the expansion tank to max. Capped it and went to test drive it.

Test drive had no AC on, lo temp, fan on lowish speed, and I drove it all over the place. Thermostat opened at normal temp, drove around fine, no issues, and no air in cooling system as the reservoir level never changed. When I got close to home I decided to turn on the heater to max, but mid speed just to see if that was the issue originally. Drove it for a couple miles then turned off the heater (had great heat too). Got to a light I was at for maybe 60-80sec or so, and started driving off and the temps shot up again to around 108-110C. Looked at expansion tank and it was about 2/3-3/4 full again. Bled out some air from the vent screw and nursed it home slow. It's off now, and dark, I don't feel like messing with it any more tonight since I have to be up early for work.


My question: Does coolant only flow through the heater core when its on? If so, I'm feeling like I have a pinhole leak somewhere like maybe the heater core which is introducing air into the cooling system. Not sure if the wrong coolant being used before I bought it has pitted the core or something. OR does coolant always flow through the core even when the heater is off? If it's THIS, then why am I only getting air in the system when the heater is on? I probably drove 60-75 miles and saw no issues with my coolant temp, or expansion tank level until shortly after I turned my heater on. Also, is there any other place I should be looking for leaks or holes in the heater area?

tl;dr: feel like my heater core or lines have a leak that's bringing air into my cooling system. At my wits end because I've replaced my radiator, thermostat housing, and water pump in the last year. Just want to drive the car.
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