I have a 1991 Cavalier. I am getting no hot water into the heater core.
last week I did!! What controls this "a valve??" electric or vacuum?? and where may I find it? What else may be the problem. Thank you for your prompt help....freezing in the mountains of Tennessee.

First, how do you know there's no hot water getting there, did you feel the heater hoses? Coolant passes through the heater core all the time, there's no valve or anything. I would suspect a stopped up core if the heater hoses stay cool.

happened to me when i ran out of coolant. but i'm sure you've already checked that(or at least i hope)
it could be your water pump too. if i was you i'd look at the hoses first though for any blockage

you woulda had a blown head gasket by now if no water was goin threw the heater hoses

make sure you have a good quality thermostat from napa, the spring in the cheap ones,motorad for example is too weak, when the engine revs the water pushes it open and circulates thru the rad whether its warm or not,overcooling the engine,thus no heat