Gasoline vapor deposits left in fuel injector nozzles after an engine is shut down form hard varnish. Over time, it can build up and clog the injectors. Gasoline additives and detergents already mixed into the gasoline you buy help reduce this problem, but they can’t eliminate it entirely. Injectors periodically need to be cleaned. Some experts recommend cleaning the injectors every 25,000 to 30,000 miles to keep them flowing at peak efficiency.

Dirty fuel injectors could cause your vehicle to be sluggish, maybe black exhaust,poor fuel economy,sometimes maybe a miss or splutter in engine.

This is the basic procedure:

1. Either disable the fuel pump and plug the fuel return line (doing this on some cars may set a fault code which must be cleared after the job is completed), or
2. Install a U-tube so the fuel will return directly to the tank.
3. Disconnect the pressure regulator.
4. Connect the cleaning kit to the fuel port on the fuel rail.
5. Remove the fuel cap from the gas tank to ensure that excessive pressure does not build up in the system. Cleaning kits deliver a large burst of pressure to the fuel system.
6. Turn the ignition to ensure that, with the fuel-pump shut off, the engine will not turn over.
7. Open and continue to open the valve on the kit until the fuel pressure reading matches the one appropriate for your vehicle. (See your vehicle’s owners or service manual.)
8. Start the engine to run the cleaning solvent through the injectors and let it run until the solvent is exhausted in about five minutes and the engine shuts down.
9. Reset the fuel-pump switch and replace the gas cap.
10. Reconnect the pressure regulator.
11. Start the engine and check the fuel injectors for proper operation using an extra-long screwdriver as a “stethoscope.” Listen for clicking sounds in rapid sequence to indicate a properly operating injector.

Cautions:

1. Badly clogged injectors may not pass enough solvent during a normal cleaning cycle to be thoroughly cleaned.
2. You may have to do some additional tests such as checking HC and CO levels in your emissions or an injector pressure drop test to confirm that the injectors responded well enough to your cleaning efforts.
3. Strong solvents may attack rubber and plastic components in the fuel pump, regulator, and fuel lines.
4. Because of the flammability of the gasoline and solvent you’re working with and the fact your engine needs to be running during this process there is some risk of injury attached to using this method.

Clogged Injector vs Clean injector

Another photo of a clogged injector vs a clean one