I'd look into getting a special coolant funnel. They have the specific caps you need and it will ensure you get the air out of the system. Make sure on a cold start beforehand check and top off all fluids in radiator and overflow and the reservoir. Then turn heat to high, then fill funnel attached to RADIATOR, and start the car till the fans kick on, then shut the car off and put your cap on the radiator and let it cool down completely cold, then top off the reservoir, and your good to go. Make sure you squeeze the top radiator hose because it being horizontal air gets stuck in the hose. Then you should be okay. Make sure to double and triple check the coolant in the RESERVOIR, and top off in the reservoir. Don't reopen your radiator cap because that little bit of pressure, because the radiator sits lower than the reservoir the coolant will leak out. It's full on that point. And triple check your over flow and make sure the hose inside the overflow is always submerged underneath coolant or else you will suck air into the system that way as well. Never let overflows run dry. that's all I can think of and if that doesn't solve your issue then I'd pull the thermostat before thinking the worst. Always try the easy stuff first before you have to stress over head gaskets. I hope the best for you.
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