Truck's hot hair blows cold when it idles or I let off the gas while driving?

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stealthassassyn

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1999 Olds Bravada AWD. 4.3L HO. Replaced the water pump for a similar problem two years ago, thermostat last year, had a problem with coolant leaking/over heating last winter, all problems have been solved, except why my trucks nniiiicceee hot air that blows while I'm driving, goes away when I let off the pedal, then comes back on as I get back up to speed, and doesn't blow hot at all when it's parked and idling?
 
Are you sure the coolant is full? Your engine is WELL known for lower intake gaskets leaking coolant.

You can also try to backflush the heater core with a garden hose or compressed air
 
Sounds like you might have a blockage in your heater hoses or in the heater core itself. Undo both heater hoses & see if you can force water through through them with a garden hose but make sure the heater is on. Hope this warms you up!
 
Your thermostat might not be openning all the way up,so while your driving down the road your engine is building up pressure to push the water through the heater core. If your vehicle has a control valve in the heater hose it might not be working just the way it should.
 
Your car has a blend door that mixes outside air with air going through the heater core to control temperature. That door may be vacuum operated and if so it could be a problem with engine vacuum or a leak in the actuator that positions the door. I hope I've given you a place to start looking.
 
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