on my 2001 monte it was and still over heating i put a water pump on today and started driving 15 to 20 min later it started over heating again. i did everything i can think of. Thermostat,water pump, i bled it fan relays,
Did you try to replace the heater core? I don't want to scare you,but it could be a head gasket as well.
Have you checked your anti-freeze? There's a good chance it might be low or empty. Otherwise, it could be either the heater core or a blown head gasket as well. Only thing I could think of in either case is to look, both are costly repairs.
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