My 99 Blazer has been showing the P0171 code for a few days. Tonight it stumbled and almost died but picked back up. I got home pulled out the cheapo scanner and got the P0171 code and the P0174. I started poking around a little bit and I noticed the vacuum line that runs to the driverside fender was completely off. I had some new line left over from when I replaced it all on my 99 p/u. I cleared the code and Ill see what happens tomorrow. Could this be at all possible why I was getting those codes? My luck its not.

 

Further investigation. I finally got some time to look at it in the light without flashlights I found a couple more vacuum leaks. I put everything back on and it is even idleing better. So we will see if the SES light comes back on. hopefully not

 

I have the same exact problem I've been throwing parts at for months. I've never done vacuum lines before. How would I go about checking them for leaks? Also, did that correct your codes? I have already replaced; plugs,wires,distributor cap and rotor, MAF sensor, fuel filter, O2 sensors. Does run better now with all these parts replaced, but still has a stumble and rough idle. As well as PO171&174 codes. Any help would be greatly appreciated. Starting to lose faith in Chevy. I've ALWAYS been a Chevy guy,so that's not a good thing....