Fuel injectors are a push fit into the seat/fuel rail and are held in place with rubber o-rings. After many years heat cycles, the rubber hardens and lose the ability to seal properly. The result is unmetered air leaking into the engine which causes rough idle, throws off emissions, and can create a dangerous lean condition. There's only one cure and it happens to be cheap and easy. Replace them!
Very leaky rings can be detected easily. Spray carb cleaner from an aerosol can at each injector's base. If the idle changes in any way, then you know the cleaner has got into the cylinder and the seal is therefore defective. It's a fine idea to swap out rings whenever older injectors are pulled during service procedures. Do it while you're in there and call it a day.
Note: All OEM/factory original injectors require black-color o-rings on their bottoms. Many aftermarket injectors (as Genesis 550cc) take blue rings on the bottom only. All top rings take the black variety. Please indicate in order notes which you need. Parts priced each.