Thermostat Bypass Valve
A valve mechanism that allows a small amount of coolant to bypass the thermostat, preventing hot spots during engine warmup.
A thermostat bypass valve (or jiggle valve) is a small valve mechanism built into the flange of some engine thermostats, or an internal bypass valve located inside the thermostat housing. Its purpose is to allow a small, controlled amount of coolant or trapped air to bypass the closed thermostat disk and flow back to the radiator or bypass channel while the engine is warming up.
The bypass valve prevents localized hot spots from forming in the cylinder heads by ensuring constant, slow coolant movement even when the main thermostat is closed. Additionally, the small pin (jiggle pin) in the thermostat flange allows air trapped in the engine block to bleed past the thermostat into the upper radiator hose during a cooling system fill, preventing air locks.
When installing a new thermostat, the jiggle valve must be positioned correctly. Most manufacturers specify that the valve must be oriented at the 12 o'clock (highest) position to allow air bubbles to escape. If the bypass valve becomes clogged with rust or scale, air will remain trapped in the engine, causing localized overheating and erratic thermostat operation.