There is a GMC fuel injector for each cylinder of your engine. This is a new component on GMC trucks that has replaced the older style of carburetors to supply the appropriate amount of fuel to the combustion chambers. The cleanliness of the fuel injectors should be a top priority of any GMC owner. For this reason, the regular changing of the fuel filter is now mandatory to help keep the engine performing as expected. Once the fuel injector becomes partially clogged, the cylinder will not be supplied with a sufficient amount of fuel and the engine will run lean. If this situation is experienced for a prolonged period of time, damage to the pistons can occur. Since manually cleaning the fuel injector will only affect the outside of this component, a fuel additive must be added to the fuel periodically to clean out the inside of the injector nozzle. The exact frequency for using this additive should be determined by the driver, based on fuel source and cleanliness. A GMC fuel injector can be tested by pulling it from its socket and the engine being turned over. Care must be taken so the spray form the injector does not go in the eyes of the inspector. If the fuel is just drizzling out, then fuel injector must either be thoroughly cleaned or it should be replaced. If just one of the fuel injectors is partially clogged, then it can be expected that all of them are in the same condition and in need of replacing. Once these fuel components are replaced, the fuel filter should also be replaced to help prevent this same situation from occurring again on your GMC truck.