Running hot after partial coolant change

RustyRatRod said

It has air trapped in the system. I always let them idle with the cap off and in the yard with a pan under it. Keep an eye on the level and when it drops, fill it up and put the cap on. The problem is air gets trapped around the thermostat and keeps it from opening. If you let it sit there and idle until the coolant drops, you are forcing the thermostat to open.

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Yep, I saw this yesterday when I changes the radiator out on my car. Started it up and let it idle with the cap on fully. As it warmed up my gauge said 0 degrees. The after 10 min of idle and feeling the radiator tanks be hot I flogged the throttle to 2500 rpms and my temp need jumps to 150 then slowly rose to 180 after 5 more min.

The scary thing was my WRX. Every time I needed to pull the radiator for work, Id fill it and the overflow and drive it for 30 min watching the temp gauge be pegged at high until it burped. It was always unnerving with an open deck aluminum block and heads.