Fuel pressure regulator?

[NoHachi]

@ Heshan: if your mechanic did a flow test and one injector differed significantly then its your best bet to replace it. You could try having it cleaned ultrasonically, but not sure if that is worth the trouble/cost. I still have a hunch that all it means is that they leave the injectors in an ultrasonic bath for some time. These baths have become very cheap recently and might be of interest to you. The middle sized ones are great for cleaning fine mechanical parts like fuel rail, carbs, etc.