There are 2 heater hoses going into firewall ..one is 5/8 and the other is 3/4 ..when car is running and at operating temp feel both hoses and see if they are equal temp ...if one is colder than other you have a heater core stoped up and needs replacement ....if not the thermostate might be bad or heaer control lever is inop wherther it be auto or manual ......
could also be a blown headgasket if you have bubbles in coolant jug while engine is running
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