This rubber hose routes crankcase ventilation gases from the engine valve cover back into the intake system. It connects the breather outlet on the cylinder head cover to the intake tube, allowing the engine to recirculate blow-by gases rather than venting them to the atmosphere. A cracked or collapsed hose creates an unmetered air leak, which upsets the air-fuel ratio and can cause rough idle or increased oil consumption. The hose is formed to the original routing path so it clears adjacent components without kinking. When replacing, verify the hose ends seat fully on both connection ports and that the retaining clips engage positively.