Possible heater core help

[ottokin]

I have a 93 GrndCherokee Laredo V8. I had a leaky radiator and put some of that radiator sealer in it (a couple times over a 2 month period). Needless to say, I had a new radiator installed but I have absolutely NO HEAT. When idling, I put on the heater and I get heat for a very short time then it changes to COLD air. Could I possibly clogged the heater core or the valve that is installed between the hoses. If the heater core is shot, HOW do I replace it myself. The garage that did the radiator estimates $600 for the HCore. I just spent $950 for the radiator replacement and a fuel pump, I will not pay another $600. I need detailed instructions please or any help whatsoever for doing it myself. Thank you...

[Cam7]

Sometimes when you add stopleak it will clog your heater core. I have fixed this before on a couple of vechiles by drainiing the cooling system and unhooking the heater hose connections where it goes in to the heater core. Then using compressed air set no more than 40 lbs or a water hose blow the heater core out by putting air or water in one heater core outlet and purging the system. This should rermove any contamination from the core. If you have to replace the core it is a big job have to remove alot of parts including the Instrument pannel. Try the purging methoid first.