My heater blows cold air at idle. and warms when driving. my temperature gauge goes up when the air is blowing. What could it be?
If you haven't done this already, check your antifreeze levels of your overflow and radiator when cool. Make sure both are full. Often times, there can be an air lock preventing antifreeze from circulating through the heater core. I normally leave the coolant cap off and jack up the vehicle in the front after starting the vehicle and letting it warm up for 15-20 minutes until all of the air bubbles come out of the coolant cap. I normally squeeze the top and bottom radiator hoses along with the two smaller heater core hoses to ensure all air is bled out. If your temperature gauge goes past the normal level or never gets to the normal level, replace your thermostat.
Assuming the coolant is full, I'd take a look at the heater core & hoses. When the engine is up to temperature, both hoses should be roughly the same temperature. If one is cold and the other hot you can try flushing the heater core with a garden hose. A shop would be able to do a high pressure/chemical flush if the garden hose method fails.