The injector seal kit consists of rubber O-rings and sometimes plastic spacers used to seal the fuel injector where it seats into the fuel rail and the intake manifold. These seals must withstand high fuel pressure and extreme engine heat. If these O-rings dry out, crack, or flatten over time, they can cause dangerous fuel leaks in the engine bay (a fire hazard) or vacuum leaks that introduce unmetered air into the engine, causing a rough idle. Replacing these seals is standard procedure whenever injectors are removed for service.