Hi all - Master Tech here, but not seen this before. What is the gunk in the thermostat below from 2010 Toyota Prius? It’s everywhere - I had to pressurize system from the top to push gunk out to get coolant peacock valve on radiator to drain. Was the Toyota red coolant not changed in 212k miles? Or mixed with wrong coolant type? I’ve seen coolant head gasket ‘fixer’ before, but it was a lot different than this (but not in Toyota red coolant). Please people who know Toyotas or have seen this before. Overheat condition, low cabin heat, I believe due to mostly plugged heater core. Passed a cylinder leak down test, dry cylinders on bore scope, passed chemical head tester. I’m tempted to flush of everything - disconnect all hoses, flush them, individually flush block, radiator, and heater core…the stuff comes out relatively easy. But I don’t want to do the work if it’s head gasket fixer goop and the head gasket is gonna fail ‘again’ soon down the road.
This kind of looks like the crystalization of DEF fluid mixed in the coolant. Was there anything off about the color?