Slow loss of coolant, only when the heater is on. Set to full cold, no loss. Is there a heater core bypass that stops flow unless heat is on? Otherwise I would expect a loss regardless of temperature setting. I suspect the heater core but am confused by not loosing any when heat is off or set to full cold.

 

Welcome to the forums!

Don't believe there's a heater control valve on the system. Just runs coolant through all the time. I'm confused too.

Might be that with the air flowing through there's some sort of temperature expansion/contraction that without the air flow doesn't happen, thereby causing the small leak to open up? Metal's weird like that though, so it wouldn't surprise me... And full cold means the A/C is running, so it'll introduce cold to the heater core and make it shrink and seal... except there's supposed to be that door... but if it's not sealing...?

I don't suppose you smell coolant or are seeing dirty water coming out of the HVAC drain...?

Dunno if that helps or not. Odd result you got there. Keep us posted!

 

A/C doesn't have to be on, it works on the vent setting too..... But the metal expansion and contraction is a possibility.

I don't rrmbrr noticing a coolant smell, but I have not had any need for much heat the past few months.

I didn't think to look closely at the a/c drainage, totally ignored the fact that both would drain from the same spot.

If something obvious doesn't show up soon, I will try some ultra violet dye.

It is baffling, I was hoping someone here had a similar experience.