This O-ring seals the injection pump housing, preventing fuel leakage and maintaining internal pressure for consistent fuel delivery to the injectors. It sits in the groove between the pump body and its cover, where it must compress evenly to hold the joint tight. A degraded or hardened O-ring allows air ingress or fuel seepage, which causes hard starting, erratic idle, and power loss. The original part is molded to the same cross-section and durometer as the factory seal, so it seats fully in the groove without requiring additional sealant. On reinstallation, clean the groove and verify the O-ring is not twisted before tightening the cover.