A poor running car that otherwise has a good tune is sometimes hard to diagnose. Injectors will flow fuel for many years without wear, but as the car gets over 200k miles it may be time to test them.
Use water to spray around the injectors to test for a bad seal. A momentary decrease in idle speed indicates that the seals are bad. (Bad seals also cause a lean condition, and depending on your fuel injection system can cause other problems).
An engine that is rough at idle but clears up on acceleration may have a worn injector. On LH cars just disconnect the injector plug to find the injector that is not opening correctly. Replacing a bad injector will smooth the idle and give more power on acceleration.
Injectors are sold without seals.
