I have a 1992 Jeep Wrangler that I replaced the thermostat and thermostat housing due to overheating. The engine seems to run at the right temperature now, but the heat blows only cold air. While the old themostat was still installed, it would sometime blow hot air and most of the time it would blow cold air.
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You may want to check the two water houses that feed the heater core. they both have to be hot. (they are usually located next to the thermostat housing.) If they are cold first check the coolant level. Fill up if needed, then you may have to check of there is an obstruction by the housing or the heater core is clogged up. I would first disconnect the hoses and see inside the thermostat housing with a lamp. Also blow some Air inside the heater core to see if flushing is needed. Well if those hoses are both hot then that must be a problem inside the cabin with the vents selection. Either an electric switch or cable. But there are already a few things to check first.
your heater hose or heater core could be plug, or your flap that opens inside of the heater housing may not be opening like it should. It operates off a vacum line, check all your vacum lines going into your heater housing.
any time you do not have hot water in the hoses to the in side of the vehicle, your water has quit either due to you having only water in the system and it froze in the winter and you started it . or the propeller just broke of old age. make sure u dont have a check valve in the hoses going to the heater core in side the car. valve would be under the hood somewhere in the duration of one of the two hoses, which is the feed. this should do u good
The only reason it won't blow hot air is because the water is not getting to the heater core. Check to see if you have a heater control valve in one of the two lines to the heater core. If so check to be sure that is is opening and closing. If it won't open then you get no water flow in to the heater core.
It dosen't happen often, but the air intake side of the core can become obstructed with leaves etc. If that is the case, the only way I know of to fix it is to remove the core & clean it. You did check the heater control valve to make sure it is allowing proper coolant flow through the core, right? Did you run it after refilling the system with the cap off and the heater on high to purge any air in there?
Have the cooling system pressure tested. If it holds pressure, that's a good thing. If it doesn't hold pressure, you may have a leaking head gasket. What may be happening is the crack in the head gasket could be blowing hot air into the cooling system causing it to overheat.