Medium and Heavy Trucks
Have Truck Repair Questions? Ask an Expert
Try crimping off both heater hose lines going into core with needle nose vise grips for a day or two, if the problem goes away you know its a heater core issue for sure so no time or money spent on a simple test.
I'm just following up to see if you need more assistance or had time to try my suggestion?
If you still have issue and are not loosing any coolant the ac drain may be plugged.
A simple test is run truck and ac and see if water is dripping under cab after about 10 minutes.
When running ac it takes moisture out of the air and needs to drain out of cab.
Let me know either way please