[GM man]

No problem with my heater. Mine gets hotter than hell, so much so that I have to disconnect my heater core in the warmer months or my AC can't keep up. Sounds like you may have an issue with the valve that shuts down the coolant flow to your heater core. That's a cheap fix if that's what it is. Could also be your actuator doors aren't opening/closing completely inside your unit or sealing well, causing cooler air to mix with the air going through your heater core. Those are operated with vacuum, so I'd start with the little vacuum hoses and work back from there. It's all very straight forward and easy to work on.