Trying to save a 99 Bonneville. It'* stumbling when accelerating slowly. The MAS has been replaced as well as the Oxygen sensor on the back exhaust. Replaced a couple of vacuum line connectors as well. It runs great while it'* cold, and at a steady speed, but when you accelerate slowly it stumbles and bucks. It doesn't do it when you accelerate hard. Any ideas would be a great help!

Is the check engine light on? Also how old are the spark plugs and wires

i would think plugs and wires too, the original wires have cyl # markings on them so if you got those they are original.

The wires have been replaced, new plugs. looks like 2 new coils, new O2 sensor, rebuilt MAF sensor, new fuel filter. It'* throwing a po171 and a po300 code. looked online and getting conflicting reports. It says it'* lean and a misfire problem. May have to take it to my technician?

https://www.gmforum.com/obd-ii-trouble-shooting-153/p0171-284036/
a few other things you can check yourself. did you check the fuel pressure, and fuel pressure regulator you can take the tube off and smell for gas, if you smell gas its bad.

id check the hose going to your pcv valve, and maybe run some seafoam thru ur vacuum hoses. couldnt hurt anything.

I did check for gas in the vacuum tube, a very faint smell of gas. Nothing leaking out, like I've heard about. Figured a faint smell would be normal. Where'* the PCV valve? Can't seem to locate it. As for the gas pressure, I don't have one. May have to borrow from the mechanic.

if you smell gas it means gas is getting where it shouldnt be. i think autozone will let you borrow one

Found the PCV valve, it rattles, but, it was coated in oil. cleaned it and put it back. we'll see. Hmmm, now what?

Thanks to all who helped!