I recently replaced my thermostat thinking that was the problem with my car. it hasn't overheated yet, but the car still blows cold air only, unless I'm going over 30mph (I'm usually speeding so this doesn't happen often) But its annoying and occasionally my car overheats on longer drives. I have a feeling it might be my heater core. Can anyone confirm this theory for me? And if anyone has experience replacing a heater core, can you provide some help and insight , I have looked everywhere and cant find a schematic or video on how to do this. I can take my center console apart most of the way, I have done it before to replace my head unit for an aftermarket stereo, but that's as far as I can get.

 

The heater core has nothing to do with the cooling of the engine during normal operation, though it can be used to cool the engine a little bit more if there's an obstruction preventing proper coolant flow through the radiator or thermostat.

Overheating plus new thermostat and cold air coming from the heater leads me to think that the problem is air in the cooling system. There's a bleed valve on the inlet line to the heater core, just behind the engine near the battery compartment. Start the car, let it warm up, and open that until a solid stream of coolant comes out of it and that should remove the air from the system. You may need to do this more than once, letting the car cool down for an hour or two between. Be sure the coolant overflow tank has plenty of coolant in it the entire time because it will be drawing coolant from that bottle when it cools down.

If you drain the coolant from the car and don't do this, the heater won't work and the car can overheat.

 

I have bleed the system several times, while the car is running and sitting at normal operating temperature, and while revving the engine. There isn't anymore air in the system that i know of, yet the car is still overheating. Could there be a blockage somewhere? could my water pump not be pumping enough through to cool the engine due to a blockage in the system somewhere?

 

super common issue that's been discussed many times. i had the cold air problem when i bought my car used (it would blow warm-ish at higher rpm) and had the overheating problem after i drained/refilled the cooling system for some service. i tried bleeding it repeatedly using the bleeder valve but couldn't get all the air out. i ended up buying a lisle funnel and haven't had problems since. it's not the only way to get the air out but it does work. there are lots and lots of threads about this. good luck.

 

Are both hoses hot? You can do a block test for about 20 bucks from Autozone. You put a chemical solution and suck air in through the radiator cap opening. If it changes color then your head gasket is shot and if it doesn't then it means there is a problem else where in the system.

Are both hoses hot when the car is at normal temp?
Does it overheat when you are idling?
You can try back flushing the radiator and heater core.
It could be the water pump, but they are pain to check and usually fail due to the bearings. When that happens you'll hear it. That is what happened when mine started to go out.