95 Chevy Monte Carlo 3100 v-6 , runs rich , hesitates,stumbles but when check engine lights up it runs fine.I do have a passenger side wheel bearing starting to howl.200,000 miles on car.plugs and wires were installed 8000 miles ago.Has ran great until now, its been like this for 300 miles,start fine, bad gas millage cuz it running rich.No other lights on,Runs rough until lightturns on than it fine.

 

you should take it and get it scanned when the check engine light kicks in...sounds like something like an o2 sensor or possibly cat converter...

worth a try to go get it scanned then you'll know whats wrong

 

we also had a blaizer doign this over here in the club. turned out to be the MAF sensor going out on it also.
so have it scaned and find out if they can narrow it down for ya.
Good luck and please let us knwo what you find out on it

 

my previous car (94 beretta) did the exact same thing ( the poor milage, running rich,etc....). my problem was the MAP sensor. But i would take it and get it scanned to be sure.I belive that autozone can do it for free.

 

Had the car scaned and the 02 sencer was bad,autozonecould not scan it cuz its not aOBD 2.Had to go to local service center.They said the PCM was givinig weird codes and needed changed .After the PCM was changed and updated, than it gave them the 02 sencerreading.$425.00US dollars .I'm wondering if it only needed the o2 sencer.How often do the PCM go bad?Run great know.