Originally Posted by BoostinZX

No worries, just wish i could figure out what the source is.. i'll find it eventually ... going to try something i should have tried a while ago, swapping the front A/F Sensors to see if the code changes banks.. long shot but worth a shot i guess to see if i have a faulty sensor...

Originally Posted by BoostinZX

Just another update. I finally got the chance to swap the A/F sensors... low and behold the problem moved to Bank 1. So it looks as though i have a faulty A/F sensor, although I replaced less than a year ago. The Short Term Fuel Trim changed to a very high number on bank 1 vs. bank 2 that it had always been on during my monitoring. I have a bad feeling its not the complete problem, or may have been caused by the deeper problem, but for now, i'm going to swap out that A/F sensor and see if that clears up the issues. I will report back tomorrow!

Originally Posted by Nostromo

To be clear (in my mind anyway), you swapped the upstream A/F sensors as opposed to the downstream O2 sensors, correct?

Originally Posted by BoostinZX

I'm hoping someone here can shine some light on some issues my wife's '05 V8 T4R is experiencing.

About 8 months ago, it started throwing codes and smelling very strong through the exhaust. (I can't remember what the codes were now, but pretty sure it was P0171 and P0173 possibly) but it led me to changing the upstream O2 sensors, gas tank cap, as well as the MAF sensor.. The gas tank cap didn't change anything, MAF didn't seem to help, but once i changed the upstream o2 sensors, it seemed to help. We drove it for a while like this and the exhaust smell was not nearly as strong. Then about 3 months ago when we got the vacuum cleaner sound under the hood during cold startup (secondary air injection system failure). I purchased the bypass kit (without plates) from slyfox here on the forum and installed it, and it bypassed the system. I was getting error codes that said stuck closed, so thought i was in the clear. Car seemed to run great until the past 2 weeks. Now we have the following.

P0174 Code (keeps coming back after resetting it)
Rough Idle (like an intermittent miss). Especially bad when in gear at a stoplight
Hesitation when you accelerate with slight throttle
Very strong gas/un-burnt fuel like odor from the exhaust, not rotten egg smell.
The car runs perfect (or seems to when under full throttle). Also seems to run fine or much better until the car gets up to temperature.

I checked for vacuum leaks by spraying carb cleaner around all the vacuum lines i could find, cleaned throttle body, ran fuel system cleaner through it a few times, checked the spark plugs (they look normal), switched to 100% premium gas and nothing has helped.

EDIT:
I have also added the secondary air injection bypass plates to try and see if they may have been slightly stuck "open" and thought that would 100% isolate it being that system from being the culprit, and the issue still remains. After installing the plates, we drove the car for about 5 miles and it ran perfectly, then right as we got home it started doing it again.

EDIT 7/21/16:
I rented a fuel pressure tester today. Car holds a steady 42psi of fuel pressure, even when the engine starts running rough after it warms up and throws the p0174 code. I let it idle and ran it up and down to 2500rpms a few times and no change... so Fuel Pressure looks good.. It also held good pressure for up to 10-15 minutes after I turned the car off. It only ever dropped down to 40psi... So I guess I can rule out the fuel pump and pressure regulator/etc..

Any ideas? I have read through a bunch of threads and some think it could be injector or fuel pump related. Has anyone had a similar issue or any suggestion on where to look next? Any help is greatly appreciated!

EDIT 8/11/16:
I figured out the problem. It was a faulty Upstream A/F Sensor that I had replaced a year ago. I swapped the sensors from side to side and the problem swapped sides. Replaced the sensor and all is good now. Hope this thread helps someone else out!