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The Inner Workings Of Fuel Injectors

The only cure for a dead, leaky or defective injector is to replace it with a new or remanufactured injector.


Fuel injectors are the spray nozzles that deliver fuel to the engine. In multi-port injected engines, there is one injector for each cylinder. On older throttle-body EFI systems, one or two injectors mounted in the throttle body supply fuel for all of the cylinders. Another variant is GM’s Central Point Injection (CPI) system, which has a single “Maxi” injector that routes fuel to mechanical poppet valve injectors at each cylinder.


Fuel flows through an electronic fuel injector when the solenoid inside the injector is energized by the powertrain control module (PCM). The strong magnetic field created by the current pulls opens the injector valve allowing fuel to spray out of the injector nozzle. When the PCM cuts off the current to the injector, the spring-loaded valve snaps shut and the flow of fuel stops.

The on and off cycling of the injectors produces a buzzing noise while the engine is running. The average on-time of an injector isn’t very long: only a couple of milliseconds at idle. At wide-open throttle, the on-time might jump to 15 milliseconds or more. By carefully controlling the duration (on-time) of each injector, the PCM can regulate fuel delivery and the air/fuel mixture to optimize fuel economy, emissions and performance.


As long as the injectors are all working properly and are clean (no varnish buildup to restrict the spray nozzles), the engine runs smoothly and delivers peak performance and fuel economy. But if varnish builds up in the nozzles over time, it can restrict fuel delivery and may even cause one or more cylinders to experience lean misfire. Adding a bottle of fuel injector cleaner to the fuel tank can help clean dirty fuel injectors and prevent the buildup of deposits. But if one or two treatments with a cleaner product fails to restore normal operation, the injectors may have to be professionally cleaned or replaced.


Injectors can sometimes fail as a result of internal corrosion or an electrical short or open. The close tolerances of the injector valve can also wear over time, causing detrimental changes in the way the injector functions. The only cure for a dead, leaky or defective injector is to replace it with a new or remanufactured injector.

If one or more high-mileage injectors have failed, the best advice is to replace the entire set of injectors at the same time. This will prevent further troubles down the road.

Other items that should also be inspected and replaced as needed include all fuel lines, the fuel filter and fuel pressure regulator. The fuel pressure regulator controls fuel pressure to the injectors. The regulator is usually located on the engine fuel supply rail, but is part of the fuel pump module assembly inside the fuel tank on vehicles that have a “returnless” EFI system. A weak or leaky fuel pressure regulator can cause a drop in fuel pressure that will hurt engine performance.

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