In late-model cars and trucks, fuel injection combines pressurised fuel from a fuel rail with air and delivers this mix to the engine. Fuel injectors are electronically powered valves that "inject" or dispense this mixture to the intake valves via a small nozzle which vaporises the fuel, allowing it to burn smoother. Gasoline left in fuel injector nozzles when an engine is shut down can build up over time, clogging the injectors. This results in poor engine performance because the right amount of fuel cannot pass through the injector nozzles. There are a few telltale signs that point to dirty fuel injectors.
One sign fuel injectors need cleaning is rough idle. Rough idle is caused when fuel injectors become clogged and deprive the engine cylinder of fuel. The engine may sound like it is about to quit, which is a result of poor fuel delivery and a sign the fuel injectors need cleaning.
Ignition misfire (when the car doesn't start in one or more tries) is another sign of dirty fuel injectors. Clogged fuel injectors interfere with the proper air and fuel mixture needed to start cars, causing ignition or "lean" misfire.
Poor acceleration is another sign fuel injectors need cleaning. Fuel cannot be sprayed properly into engines if fuel injectors are clogged. When accelerating, the engine needs more fuel, and dirty injectors cannot supply the proper amount, which results in stumbling or hesitation when accelerating.