Computer controlled engines and fuel management systems lower emissions, improve gas mileage and improve drive-ability. Drive-ability refers to how the car eliminates problems like stalling, hesitation, rough idling, and hard starts.
First introduced in the 1990 Subaru, fuel injection systems are a big improvement over carburetors, which often had problems like flooded carburetors, needing to pump the accelerator, hard starts on cold mornings or hot afternoons, re-jetting for different altitudes, and vapor lock. These drivability problems have virtually disappeared with the advent of fuel injection. Today, most engines use computer-controlled fuel injection.
Each engine cylinder has an individual injector, which sprays fuel right at the intake valve. The injectors spray fuel in a fine mist that vaporizes to power the most efficient combustion. Dirty fuel can lead to drive-ability problems because they don't produce the fine spray needed for complete combustion. They can stick "closed" and not supply any fuel, they can spray more fuel off-centre in the valve, leak fuel, or deliver uneven amounts of fuel to each of the cylinders (this last one can lead and engine misfiring) . With today's computerized engines, a missing cylinder causes the engine to run even rougher, because its unused oxygen goes out with the exhaust, which causes the oxygen sensor in the exhaust to think the engine is running lean. The computer then signals the (other, working) injectors to stay open longer to add more fuel, which compounds the misfiring problem and increases emissions.
Gasoline today is designed for fuel injection systems, and burns very clean. Fuel filters trap virtually all the dirt particles down to the 10-micron size. The filters work less efficiently at low speeds (due to the lower fuel flow rate) leaving some dirt end up in the injector tips.
Watch out for sign of gum deposits, which can also create drive-ability problems. When a hot engine is turned off fuel, the injectors are "heat soaked" and let the remaining fuel evaporate on injector nozzle tips, which leaves a gummy residue on the nozzles which can develop into an off-center spray pattern leading to drivability problems.
One simple way to clean out deposits in injector nozzle tips is an "Italian Tune-Up," where you drive on an open stretch of highway with a few full throttle accelerations (and of course, observing speed limits). You can also use a commercial fuel injector, typically adding one container to a full tank of gasoline. Don't over-add since their chemicals can damage rubber components in the fuel system. You can do this as preventative maintenance as recommended in your Owner's Manual or on the container. These additives are especially helpful if you do mostly grid-locked rush-hour driving.
If this doesn't work, take your vehicle to a shop that specializes in servicing fuel-injection systems. They disconnnect the fuel injector from the fuel the system and re-connect them to a tank containing a detergent cleaning solution, which also cleans the injectors, the engine, as well as any carbon deposits on the backside of the intake valves. This creates a smoother flow of air-fuel mixture into the cylinders. If this doesn't work, then the injectors can be removed from the engine and cleaned, or they may have to be replaced. You should replace all the injectors together, since they tend to have problems together, some just a bit sooner than the other injectors. Often, it's more cost -effective to replace injectors rather than having them cleaned, only to find that they have to be replaced.