12.7 Detroit Coolant Issue

OK I posted a question about this truck several months ago. Customer came in and the pressure cap was chirping and I did a compression leak detector on it and the fluid turned colors. I told him he probably had a head gasket issue. He left and put Blue Devil in it. Then it started running hot so I flushed the system and replaced the thermostats. I also removed a clog from the 1 1/4 hose that runs from the surge tank to the top of water pump. I removed one of the lines to heater core while truck was running to make sure water pump was pumping and it was pushing out like a hydrant.
Now that it is getting cold the truck only has heat when he is running down the road or has truck at high idle. If he stops no heat. He said that the bottom radiator hose is cold when idling and he can open the drain valve and the water is cold and the temp gauge shows 180. Also one half of radiator is hot and the other cold. This is after coming out of Florida into SC.
I am thinking either the system is getting air pockets or the radiator is clogged. He says that since I removed the clog on that pipe and replacing thermostat and flush it isn't pushing coolant. I had him put a water bottle on the small hose off of surge tank and there wasn't any bubbles. I have no idea. He is supposed to bring to me tomorrow.

 

No leaking from water pump.

 

Head gasket sealer. From what I have read it can mess up the entire cooling system especially if you don't follow instructions. He did not.
I would think if the water pump was messing up that when I cracked open that heater core cut-off valve with the line off and it shot water 10 feet that the pump was working. I have only ever seen one impeller issue on a Detroit before. I guess I could order one and take his off and replace if I see issues. Darn things are so expensive now though. I remember when they were uder 200.00. Now they are around 500.00.
https://gobdp.com/product/head-gasket-sealer-38386/

 

Yeah I always check that line when I am putting coolant back into a truck and if I have the thermostat housing off. That one can cause some issues.

 

I do not remember. Just know it shot out with force.