update: the buyer had the mechanic follow the instructions on flushing the core. Everything went perfectly. Then, he called and said the temp gauge had not been coming up to the midpoint no matter how long he drove it. "The thermostat is stuck open," I nearly screamed into the phone. It would have saved a lot of time if he had told me that to begin with. Took it back to the mechanic, changed the thermostat, problem solved. The dealer wanted $1700 or so to pull the dash, etc.