Some models have an inline fuel filter. Usually it is either behind the fuel tank or in the engine bay. You should check the Inertai switch too as they can cause issues by failing intermitantly. I have included Fuel Pressure specifications, Fuel Filter Removal and Instal, Fuel Pressure release and Inertia Switch location.
The plugs should be a tan or brown color if it is running at proper air/fuel ratio.
You might need to run a high quality fuel system cleaner if you never have.
I reccomend BG44K as it is the only cleaner on the market that will remove deposits and clean injectors on cars that have never had a good cleaner run before. The only other cleaner on the market that truly works is SEA FOAM. You can get Sea Foam at an auto store, but BG44K is only found at service stations and Car Dealerships.
Run one can per tank and run two tanks and then one more tank to get all deposits out as they will end up in the engine oil. So, it is best to change oil after third tank. The car may perform differently as deposits are removed so give it a chance after third tank before results can truly be seen.