Engine parts, especially pistons, are designed to fit precisely when they are at normal operating temperature. Running without a thermostat greatly increases engine wear, emissions, and the formation of sludge in the oil from blow-by not being heated enough to vaporize and be drawn out.

The most common cause of a thermostat not opening is a leaking head gasket. Combustion gases pool under the thermostat and prevent it from opening. Thermostats open in response to hot liquid, not hot air. Without the thermostat in place, the coolant can circulate to the radiator and the air bubbles will get pushed out to the coolant reservoir. That's not a fix but you may be able to drive the car a little while until the problem gets worse. It won't get better. A mechanic can do a chemical test to verify the head gasket is leaking into the cooling system.

Answered by caradiodoc (expert)
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