my EG Civic with a B18C motor runs fine but while i was tuning the car a few days ago ( it was running really rich) i had it running fine but after i let it sit and started her up cold i was watchin the wideband A/F gauge and the car was sputtering really baddly and the A/F was goin from rich to lean fairly quickly. I was able to fix it with some more tuning.
How ever, my friend with a B18C in his 98 Civic called me up and told me he drove trough some water a while ago and the CEL came on, and went off, he is telling me now that the car is getting really rich around 3000 rpm and and the car is sputtering. He brought it over and i was driving it and the car started to sputter, i looked down at his AEM wideband gauge and the car was goin from rich to lean pretty quickly, similar to how mine was doing. If i am driving and accelerate the car and i look back in the mirror i see puff of dark colored smoke, telling me the car is rich.

Now if this was my car i would fix it , but on this car, that is completely stock minus the AEM wideband. i am clueless at why this car would do something like this . I have no idea if the water he drove through screwed something up and is causing this. Does anyone know what can cause this and how i may fix it?

Why would a car with stock ECU just start to go rich and lean at some rpms ?
What should i check or fix?

Car specs:
98Civic
GSR motor
AEM wideband
5 zigen exhaust
completely stock ECU
no fuel adjustments
stock timing

Need help as soon as anyone can

 

any suggestions?
anyone know why a stock car will just start to run rich?

 

bad narrow band o2 sensors sending the wrong signal to the ecu.

change the sensor on it.
also check tps voltage

 

Check plug wires and spark plugs as well, if theres a cyl not getting spark the fuel will just go out the exhaust and cause the wideband to do that.