Little engine overheats unless the heater is on. Wierd

This is Gary here. I just joined your forum. I have a puzzler. I am a mechanic. I got this 2000 tacoma and it overheated to begin with. I went ahead and installed a new head gasket. It runs just fine if you have the heater on and fan on high. It supposedly has a new radiator. As soon as you shut down the heater it will overheat and boil into the overflow. Do you have to purge the air out of the radiator or do I have a bad water pump, or what? I dont get stumped too often but I am human. I thought about purchasing a engine block tester but my thinking is if it does fine with the heater on then it cant be the new head gasket. Anybody got a clue? By the way, I think I can get used to this forum. Its fun. I recently joined a bmw forum and got alot of answers from the real world people.
Please help me pppppppppppplease?
Signed
Garysquest

 

thermostat.... only thing left

 

check the t-stat or maybe a clogged radiator

 

Yeah I would suggest a bad thermostat or you have a air bubble in the system?

 

thermostat? flush the radiator?

 

I'd say bleed it first, especially if it's V6. T-stat second. Block tester third.

We rarely ever Taco's in with over heating concerns, so it's probably something simple to find.

Fellow knuckle-dragger here, glad to see I'm not alone!