I say try the new T-stat and rad cap and see what happens. Make sure you’re not sucking your lower rad hose flat. Also make sure your temp selector is on max heat to push any air trapped in the heater core into the rad.

Make sure your hose between the rad and overflow tank is not blocked and is tight on the barbs.

If your cooling system is operating properly the air should naturally purge into the expansion tank after around three drives.

Driving around, any air in the system will collect in the upper radiator tank.

When the engine cools off the rad caps vent valve relaxes. When you start the engine the water volume expends and that will push any air collected at the top of the rad into the expansion tank via the vent valve in the center of the cap. That valve is temperature sensitive. So when hot coolant starts going through the vent it closes to allow the cooling system to build pressure.

All three 3gen 4Runners I bought had a broken rad cap. I’ve had a Napa cap in my 97 for 3+ years without any issues. But, saying that everything is new. Engine, rad, etc.


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