Hello,
I have an 05' Monte Carlo with about 150k miles on it, and it has just started throwing a Bank 1 Lean code P0171. Looking at the freeze frame data, it only throws when the car is sitting idle. The car also struggles to cold start sometimes stalling out. I replaced the engine air filter, also checked and cleaned O2 sensor, MAF sensor, and checked for leaks to no avail. Any Suggestions?

 

Are the fuel trims always skewed well off zero, but only far enough off to trip the code at idle? What happens if you unplug the MAF and let it start up on MAP only - does it run OK then?

I know you said you checked for leaks, but a vac leak is one of the few likely causes of what you're seeing. A small leak could represent a high % of the total airflow volume at idle (as the engine is consuming little, but at high vacuum levels due to the closed TB - tripping the code when it hits the max fuel add %), but once the rpm goes up and vac level goes down, it becomes less and less impactful as so much more air is coming through the open TB (but would still keep trims skewed in that direction, just at lower and lower trim amounts with increasing TP%.

 

Fuel Trims return to what I presume to be normal while driving. Long term stays at 0% and Short Term does not go past +-5%. I will try unplugging the MAF sensor sometime this week as well as doing a more in-depth look for a small leak. Is there a better method other than soapy water?

 

Interesting, I've not tried soapy water. Given it's sucking in, I'd think it might be hard to spot (vs a pressurized system like a natural gas line that bubbles outward). I know I've heard of people using water to listen for the sucking sound, I've just never tried it personally.

My favorite is smoke - small smoke machine w/ hose, cigarette, incense - anything you can concentrate in a softball size region without clouding up the whole engine bay. Its really nice for pinpointing stuff that may be out of view (the bottom of a hose barb, etc) as all of the local smoke gets pulled into the hole. Certainly there's some risk with the less official methods given you're adding an ignition source, but as long as you're not actively leaking fuel/vapor, it's ok.

I know lots of folks that like to use external fuel sources - can of starter fluid, unlit propane torch, etc. Tends to be a more apparent signal when you've found it as the engine rpm will climb. I just feel it's riskier putting a bunch of combustible fuel on the outside of the engine - all it takes is a cracked plug wire or boot to turn into a real bad day.

 

I don't know how qualified I am to say anything or if I'm even correct but my car had very similar issues and I replaced the fuel pressure regulator and they all went away. You can check if its bad by pulling the vacuum line leading from the fuel pressure regulator off when you're having trouble starting it, and if there is fuel present then its bad.