what causes the fuel to run to rich? i have replaced the starter,fuel pump,and on 1991 Oldsmobile Cutlass Ciera

cat.converter. will run for a while then die. a mechanic told me that the gas is running to rich and might be an electrical problem?? any help??

First guess it would be an O2 sensor.
It would appear you are willing to spend this amount of money and not fix it. What the hell replace the O2.
I'm sorry, Take it to a PROFESSIONAL AUTO TECH (PAT) to look at the serial data stream.
Stop wasting money!