Hi guys...did a search on this topic without success, so hopefully I'm not "that guy".

2000 Tahoe, 5.3, 4wd, auto temp control for each side, rear heat/AC module. Owned it since new.

Engine temp is right at normal...about 190F.

I've replaced the water pump and t-stat already (Yesterday.) Had a little seepage, so knew it was time. When I replaced it I blew compressed air through the heater core lines and they appeared to be clear. (Blew from the water pump hoses connections back through cores, then from the fitting before the Tees at the firewall. I did not blow through each core separately, though....connectors looked pretty fragile, and I was afraid I'd break them.) I did not back flush the heater cores with water, since I figured they were clear if air got through.

Felt the heater feed lines from the water pump to the firewall with engine running, heater on. One is really hot, the other not so hot.

Inside, air blows normally through the front and rear systems, just cold air in both. Thought it might be a door actuator (changed them all in my 2005 Denali) but I'm getting a full stream of air...just cold air. And, it's doing the same thing in the front and rear, which is the head scratcher... I'd be surprised if both temp actuators in the front and rear systems are bad, but I guess they could be.

BTW, no fault codes from my OBD scanner.

What did I miss?

 

More info: I can cycle through the different vents inside in both front and rear, so I think the actuator door that controls the vents outlets is OK. If I turn the temp setting control full hot or full cod, nothing changes in either F or R.

 

Any signs of dex cool corrosion around inlet /outlet on heater core(s)?
Could still be plugged enough to not to transfer heat .