I repeated the same test as above...this time using
mineral spirits...
Attachment 20349
The results were slightly better than paint thinner. After 48 hours of sitting in mineral spirits, the flux was mostly dissolved and it completely fell apart and disappeared with some minor shaking of the jar.
If this goo is petroleum-based (like flux) then mineral spirits
should break it up and cause it to fall off of the surfaces of the heater core where it has deposited. At that point, you would still need to flush the core to remove the dissolved goo.
I will probably try this on my car once the weather warms up. Using a method similar to
what Doax did, I would fill the heater core with mineral spirits and let it sit for at least 48 hours. Then I would fill a 5-gallon bucket with hot, soapy water and pump it through the core (in both directions) to clean out the residual dissolved goo.
It sounds good in theory and it won't take a ton of time, effort or money. If it works, it could potentially save replacing a heater core.