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Old Sienna
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Many t'stats today will have a small ball check in them to let air pass by during a fill but not all. In the past where the design did not include that vent, I drilled a 3/32 hole through the t'stat
Me too, but for a different reason. Heaven forbid the head gasket leaks, any escaping fumes will vent through that small hole instead of pushing all the coolant from the engine. A new head gasket is better than a new engine, simply by having a tiny hole in the thermostat.
 
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