Coolant Bypass Pipe
A rigid metal or plastic pipe that routes coolant around the thermostat when it is closed.
A coolant bypass pipe is a small, rigid line that allows a small amount of engine coolant to circulate within the engine block and cylinder head when the thermostat is closed. When an engine is first started and is cold, the thermostat remains closed to block coolant flow to the radiator, allowing the engine to warm up quickly. The bypass pipe ensures that coolant still flows through the engine water jackets, preventing hot spots in the cylinder head and ensuring even heating.
These pipes can be made of steel, aluminum, or molded plastic. Plastic bypass pipes are particularly notorious for becoming brittle, cracking, and leaking at their connection points due to constant exposure to hot coolant.
Replacing a leaking bypass pipe often requires removing the intake manifold or other top-end engine components to gain access.