So it's an 01 s40 with 113,000 mi. i had a CEl so i pulled codes and had a mass air flow sensor code. cant remember what it was but i cleaned it and it's gone now. the other code was P0172 - Long-Term, Fuel Trim Rich. basically telling me to replace the upper o2 Sensor, i also noticed under moderate to heavy acceleration the car sort of shuddered left to right it seemed. so i replaced the 02, cleared codes, unplugged the batt for 10 min, hooked it all back up and drove ~22 mi. the cel comes back on no more mass air flow sensor code but the P0172 returned. also, while the check engine light was off the car accelerated smoothly! the shudder returned with the light...

i've searched quite a bit. a lot of people suggested a vac leak. so i visually inspected the vac lines and elbows then sprayed starting fluid all over the place to listen for an elevated idle. no luck.

so what should i do next?

 

Maybe disconnect the battery for more than 10 minutes after clearing the code? Although I don't think the O2 had any bearing on your problem, it certainly doesn't hurt to have a new one. Otherwise I'm thinking like you.

And I've always found a vacuum leak caused a lean condition on MAP equipped cars. Does a vacuum leak make a MAF car run rich?