Pontiac Repair: Car running rich, fuel pressure regulator, oxygen sensor

Question
QUESTION: Hi Todd,
I have a 1988 Pontiac grand am LE that was coming up with a code that said it was running lean. I had the fuel pump replaced, now the code says it's running rich. Any idea why? My mechanic can't seem to figure it out. The check engine light will come on when it's idle (like at a traffic light) and when I start driving again, it goes off. It's driving me crazy! And help would be greatly appreciated.
Thank you!

ANSWER: Hi Kathy:

what engine is in this a 2.3 liter or a 2.5 liter engine?  the only thing that was changed on this was a fuel pump and it went from a lean code to a rich code? It's possible that the wrong fuel pump may have been put in this and it is producing too much fuel pressure and the pressure regulator can't handle it and the engine is getting too much fuel now.. If that is all that was changed that is the only logical guess. The next guess would be possibly a bad oxygen sensor... But again it's just a guess and without seeing it it's just that a guess. Good luck :)


---------- FOLLOW-UP ----------

QUESTION: Hi Todd,
Thank you for getting back to me so quickly. The engine is a 2.5. And, yes, the only thing changed was the fuel pump. When it started running rich, we changed the oxygen sensor, but that didn't do any good. Do you think changing the fuel pressure regulator would help?
Thank you!

Answer
I doubt that will fix it. I would double check to make sure the right pump was put in this and an original equipment pump not an aftermarket one. what really needs to be done is have the fuel pressure checked with a guage and see what the pump is putting out. If it is with in specs which in memory serves me right would be between 9-13 PSI???? if it's too high it can be the pump or the regulator... If it's withing those #'s then a scan tool will be needed to see what the engine data sensors are doing to make the engine run rich.