Hello everyone. New to the forum and just bought a 97 4.6 150. Have a slight problem

When i bought it, there was NO heat at all. Did a full coolant flush and looks like there was stop leak in there. Rinsed it out about 5 times and hooked up a hot water hose to the inlet of the heater core. That was clogged pretty good, but it blew threw. Now the heat works on low, but only after like 20 min of the truck running. As soon as i turn the heat on (panel or defrost), the coolant temp shoots to H in about 2-3 seconds, comes back down, then shoots right back up again and stays. If i turn the fan off, the temp drops back down after about a min, and its good... Thermostat? its new. Fan clutch feels fine. I dont understand how the temperature could change so much so fast.

Any help with this would be very appreciated. Thanks

 

Could be a problem with the temperature sending unit.

 

IF the thermostat came from Autozone....Pitch it.Get a real one from NAPA,otherwise,have you actually seen an overheat condition(or is it just indicating so?).....IF its not the thermostat,Id look at temp sensor as previously suggested....

 

make sure the overflow tank is topped off after you get the air bubbles out of the system.