My son's 97 Sable with 3.0 pushrod engine, overheated on startup after a long drive then being parked for half an hour or so. After an overnight cooldown, the gauge would read hot and the cap would begin to spew within a few minutes of running. I took out the thermostat and confirmed it was bad by boiling it on the kitchen stove. (At a rolling boil, it would open maybe a sixteenth of an inch at most.)
I flushed out the old coolant, which was brown, and ran it for a week with no thermostat and a bottle of Prestone super radiator cleaner in it, then flushed all that out. I isolated the heater circuit from the engine and ran lots of water through the core in both directions.
With everything back together and a new thermostat installed, it runs fine, the temperature gauge reads normal, but there's no heat. (The heater worked normally before the thermostat problem.) The heater hoses are hot to within an inch or so of the core fittings, the bypass hose is hot, but the core metal is cold. What could be going on here? It sounds like a core blockage, but each time I've flushed the core, it's appeared to have plenty of flow.