A common problem with fuel injectors is the buildup of fuel varnish deposits in the nozzle that restricts fuel flow and/or disrupts the injector’s spray pattern.
It doesn’t take much of a restriction in an injector to lean out the fuel mixture. Only an 8% to 10% restriction in a single fuel injector can be enough to upset the air/fuel mixture and cause a misfire.
Gasoline contains waxy compounds that can leave varnish deposits in the injectors when the fuel evaporates. These deposits tend to form after the engine is shut off.
Heat from the engine causes residual fuel in the injector tips to evaporate, leaving these solid/semi-solid deposits. The next time the engine is used some or all of these deposits may be flushed away, maybe.
Gasoline is supposed to contain enough detergent to prevent these deposits from sticking and accumulating in the injectors.
The detergent levels, additive levels, vary with different fuel brands and octane ratings.
Consequently, filling up with the cheapest gas one can find may not be the best idea in the long run — especially for short trip, stop-and-go city driving that causes deposits to form at a much faster rate.
On four-cylinder engines, the #2 and #3 injectors are in the hottest location and tend to clog up faster than the end injectors on cylinders #1 and #4.
The same applies to the injectors in the middle cylinders in six- and eight-cylinder engines.
The hotter the location, the more vulnerable the injector is to clogging from heat soak.
The cure for dirty injectors is to have them checked and cleaned periodically.

Spray Pattern

On many late-model engines, the shape and direction of the spray pattern is critical for clean combustion and good performance.
If the injector nozzle is dirty, the pattern may be distorted or deflected to one side, causing a lean spot in the combustion chamber that can cause misfire, or even preignition or detonation.
The Spray Pattern affects fuel atomization and fuel distribution within the charge, ie you may have the correct amount of fuel for the amount of air but the uneven distribution means pockets of lean mix and pockets of rich mix will exist in the charge.
Obviously not as good or as efficient as an evenly distributed fuel/air mix.

How Do We Know?

That is the question
While most of the injection system is monitored by the ECU for correct performance and operation, Injectors have limited monitoring.
At the moment Spray Patterns and Fuel Flow cant be monitored directly, the only monitor available is the O2 sensor which indirectly measures the effects of dirty injectors. But the O2 sensor is also indirectly measuring the effects of everything that may affect the mixture so it is only a clue.

We wont/dont know anything about the condition and performance of the injectors without removing them and having them checked off car by a specialist Injector Servicing Machine like an ASNU. This is the ONLY way to check injector performance

On car cleaning ? Injector Cleaner in the tank?
While these methods sometimes will have an effect, you have no idea of spray pattern inegrity, flow rates, either before or after you have "cleaned" them.
And if they dont do anything does that mean your Injectors are good?

There are "clues" that suggest injectors may not be performing 100%

• Poor fuel economy
• sluggish low rpm performance
• sluggish pickup
• pinking under acceleration
• overactive knock control
• loss of top end performance

While there are lots of things that may cause these problems, generally most are easy to diagnose/inspect and repair.

Just remember injectors are usually not obviously faulty.

More than one engine has had a topend rebuild when all that was wrong was dirty injectors