94 aspire heater core

I'm having a heat problem. The only time I get any heat in my cab is when the engine is reving. If it's sitting at an idle, I get nothing.
The engine is up to temp.
Does this vehicle have some sort of vacuum valve to the heater core? I don't want to take the whole dash apart for nothing.
I'd love any suggestions.
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wonger opined in

fill the coolant... on most cars, that shows air in the engine
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