I bought EGR Valve & Catalytic Converter for 2002 Pontiac Grand Prix GT 3.8L with 225k on Friday, Saturday I had it installed, I should of done the EGR myself just I'm not good with those things & didn't know if I replaced the EGR before the Catalytic Converter if that would mess anything up one before the other as I don't want to be stranded. Only thing is after those 2 things were replaced it still runs like crap. Suggestions? I've asked around. I've been told to get it hooked up to a computer. I contacted another mechanic in MN & he told me take the throttle body off & clean well & then replace.???

Get your car linked to a dynamic engine monitor by an experienced mechanic. This will give you live feedback. As Robert stated above mass air flow definitely a possibility......Mass air flow sensors are not very expensive. They are very delicate careful when installing....

It is on your intake by the filter. unplug it, remove the two screws, there is a little sensor on it held in the airstream that when it gets dirty it insulates is giving the pcm wrong readings about the actual air intake and in turn not giving the engine proper amount of fuel for the amount of air coming in. if it was malfunctioning it would set a code but being dirty it's still working just not accurate. clean off the resistors in the sensor you can use brake clean or you can also get maf cleaner made for doint that. if you're careful you can use a q-tip. reinstall and if that's the problem it should run better and probably use less fuel. just by regular maintance like that i got my 97 grand prix up to 36 mpg on the hiway.

could be anything, map, mass air sensor, throttle position, sensor idle air motor, or with the mileage you have a combination of one or all, how are the mechanics of the engine , no worn camshaft, no shot fuel injectors......possibilities are endless, and troubleshooting without the proper diagnostic equipment is a shot in dark at best