In my opinion it's unlikely that casting sand took 6 years to clog the core. That's tpically something that happens early on. If both lines are warm you have hot water flowing through the heater coil. Even if it's blocked some you have got decent flow because otherwise the return line wouldn't get warm. Could still be air in system I guess.
When you say the coolant was pinkish, it should be either all greenish or all pinkish depending on type of coolant. I'd think an 08 would have the newer coolant type but I don't know for sure. If you mixed coolant types that could be a big issue. Some of the coolant types react with each other and turn to sludge. Might be a good idea to flush the system if you think it may have happened. Just my opinion, others may be able to chime in more. If the rig wasn't overheating I'd say it could just be the temp control for your HVAC system but that wouldn't cause the rig to overheat.
