So I ran the heater with an IR thermometer and a camera that captures heat imaging. Still waiting for the camera images, but not real sure they will show much other than neat red patterns. The thermometer showed temperatures similar to what my scan gauge reads, and with the engine running at idle it won't go past about 80 degrees F on the gauge and at the heater. If I rev the engine or drive it and keep the rpm about 2500 or higher, the heater will keep running. I have tested this now a few times. So low flow it is.
I'm now reading up on the coolant system and looking for a culprit that either might be damaged, old, not working correctly, or dirty that would restrict flow other than the water pump pushing with more force. I cannot find much for diagrams and explanations on how the system works, but still looking. I did read up on the 'cooling stack' and how it can gather gook and residue, not sure if that would be enough to restrict flow, but I'm going to dig into that next.
I don't understand the function of the 'cycle valve' in this basic sketch of the cooling system that markxengineering did, and is that 'T Joint' a valve or just a 'T'? thanks