Help - '04 Monte w/ L36 - P0171 and P0131

[The_Maniac]

OK, first item on the table, I recently modded the Monte Carlo. It has been under cover and has not been ran since last October and I installed the mods about 2-3 weeks ago. I wish I would have driven the car recently pre-modded to know if this was a condition that happened while being stored for the winter vs a result of the work.

Anycase, mods I've done BEFORE I stored the car (and no issues):
L67 Coil Packs
ZZP 2.5" Downpipe with high flow cat

Mods I did as I brought the car out for spring:
ZZPerformance ER Rockers
ZZPerformance HV3 High Velocity Intake Insert
ZZPerformance Power Log
Custom L67 Throttle Body (ported inlet and port matched outlet)
Port Matched Lower Intake with Aluminum Gaskets
Tuned PCM by PCMforLess.com
NOTE: - I used my factory UIM, but swapped the stove pipe on the LIM with the one that came with the Dorman UIM gasket kit.

After getting the car together and it's maiden voyage, I kept getting a repeating P0131 (low volts on the O2 sensor). After doing some research, it appears their might have been material in the PLOG that burned off and damaged the O2 (I used the air compressor and blew some debris/dust from it, now I wish I would have ran a bottle brush through it). When i first drove the car, off the line, it would stumble some (until the gas pedal reached a certain spot, then it drove like normal).

So, OK, with about 62,000 miles on the car, I figured, regardless, I'll replace the O2 sensor (I had about 150-200 miles on it since I started driving it with the new mods). I bought a Denso upstream O2 sensor. Replaced it, no problem. Stumbling seemed to be gone, car seems to run great, but now it throws P0171 (running to lean). I have done the following:
- Using wiggling, visiual inspection and a can of starting fluid, looks for vacuum leaks, I can not find any.
- Smelled around the injectors for smells of gas while the car was running, nothing.
- Checked all wiring, nothing appears damaged or pinched
- Swapped the IAC from my L67 Throttle Body with my stock L36 Throttle Body, no change

Now, the car is throwing both P0131 AND P0171 as of Sunday. And Sunday was the first time it began to have a slight roughness in the idle (nothing huge, very slight, but enough I noticed it). But even that went away and back to normal idle the next few times I've driven the car.

I've read these problems linked to:
- Vacuum Leak (I can't find one)
- Upstream O2 sensor (I recently changed with a Denso one)
- Restricted Fuel Filter (about 5000 miles ago, I replaced the fuel filter with a genuine AC Delco one)

In addition, I finally ran the tank of gas that sat since October (with fuel stabalizer in it) and put in fresh gas (about 100 miles on that tank). Since the mods, I have about put maybe 400+ miles on the car.

I am running out of ideas and data to research. One of the most confusing things I have is 90% of the time (or more), the car feels like it's running great with no problems.

Anyone have any thoughts on the subject? Thanks!