GMC Troubleshooting Questions? Ask a Mechanic for Answers
Have you tried back flushing the heater core yet? We see these trucks in with plugged up heater cores fairly often. If the heater hoses are warm, and the blend door is working correctly then there is likely a coolant flow issue. The air volume out of the vents seems ok right?
I've seen this engine heat up the return line backwards when it's sufficiently restricted. I usually get a little concerned when the return line is just as hot as the supply line. That means that the heater core isn't taking any heat out of the fluid as it flows through. The return line should be slightly cooler than the supply line.
The engine seems to run at normal operating temp right? Not too cold?