Yes I agree, it seemed like a flow issue.

But when I flushed everything, put a new pump in (the old one looked great) and installed a new radiator I still had the same issue.

Then when I installed the thermostat everything worked fine.

I came to the conclusion that the coolant does not flow correctly without having the thermostat installed.

 

Seeing the same thing here.

Trying to troubleshoot my heater not working I removed the thermostat from the housing.

Well, the thing fell down the engine bay and of course, it did not hit the ground.

Spent a good while looking for it but it seems the truck ate it. Even went fast trough some speed bumps to see if it would drop and nothing.

It started overheating after that, so I too think it needs the thing there to direct the flow otherwise coolant don't circulate correctly.

My question is, did your heater started working after you did all this? That's what I'm chasing.

Thanks,

 

When I replaced the plastic section of the cooling system which contains the thermostat I had some trouble bleeding air out of the system. It took 3 cycles of driving, cooling and refilling the overflow tank before I got heat from the vents. My guess is you had an airlock in the system. I don't recall seeing any overheat on the Guage but perhaps the engine was running hot.