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A bad cat will normally run good at an idle, and act up at higher engine speeds. If the fuel filter was dirty/bad it should act up at higher speeds too, and idle well. I would put some injector cleaner in it, and check for vacuum leaks. After the injector cleaner change the fuel filter. Because the injector cleaner can clean some deposits out of the tank and the filter will catch them, of course it will clean the injectors too. There is no need to put any junk into a new filter, so run the cleaner first, then put a new filter on, so you will have a clean filter. To check it for vacuum leaks get a can of WD40 and spray it all around the intake, throttle body, and vacuum lines. If you hit a vacuum leak the engine will speed up for a second or two. This will allow you to find nearly impossible to find vacuum leaks. I would like to see you check the fuel pressure too. If the pressure is off, it will show up the most at an idle. Also be sure to plug in and check for trouble codes. Most auto parts stores will check the codes for no charge. It helps them sell parts, and promotes customer loyalty. The on board diagnostic system might have some info recorded that will help you track the problem down. If you have any more questions I am here to help. Good luck, and Thanks for using Just Answer.
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