Any Toyota Techs on here?

The engine temp is where it should be. I put a NEW control valve on it. I did the heater core because I thought it was the problem. I have also done the water pump and timing belt. I finally broke down and took it to a friend who is a retired Toyota tech. He is also stumped. By accident he noticed a small threaded rod that goes into a plastic arm. He adjusted it a little and heat got worse. He went the other way and it got better. He has no idea what it does. The heat has been pretty good until yesterday. Im gonna scew with that rod today. It on the passenger side under the heater box. There were no signs of corrosion in the other control valves.