07’ Yukon XL heater

This is new,
I had my engine, 5.3 liter changed. Heat worked fine beforehand, but now doesn’t. Since getting it back, I’ve done a radiator flush, this helped for about 10 minutes, then I put the cap back on. I kid you not, by the time I got back into the truck, it was blowing cold again.
I thought maybe I had to “ burp” the system some more, so I took the cap off again, kept it on high heat defrost and temperature, and ran it another 30 minutes. Same thing, within seconds after putting the cap back on, cold.
Has anyone heard of such a problem?
Is this the heater core going bad or what?
Any help will be appreciated.
WHTrouten

Might have an air pocket in the heater core. Take the heater hoses off and run a garden hose through one way, then run it back through the other.

Direction of flow may have gotten reversed with new engine install. Although it should not be a big deal-I have seen it fix your problem by reversing the hoses at the firewall.

Actually, it might make a difference. If there is air in the system it might be better to have the return line at the top of the core, so at least the pumped liquid can fill the core and eventually start some flow into the pump suction line.

If you are going to go ahead and remove the heater core hoses and flush. Put some white vinegar in it and let it sit over night. Then flush again. Most refill the core with antifreeze before putting the hoses back on. It will help to eliminate air pockets.