It'll be back, i'm sorry to say.

When you guys want to solve the code, let me know. I realize that sounds cocky and arrogant, it isn't meant that way. I do know how to help you guys find and solve this code. I've documented it many times on the forum and if you have further questions please ask. Here'* some of the most common causes.

Cobram.. Did you take the tube off the car and hold it up to light to look for the leak? This is how well that tube hides it'* leak. It requires removal.

Check battery cables for corrosion. If they are corroded , check under the rubber boots, if it'* there. Buy new cables.

Check the evap soleniod and ensure the port labeled CAN does go to the canister and not the throttle body.

Pull the ground going to the engine block and ensure it'* clean (between starter and ac compressor). Then go to the back of the engine by the oil pressure sender and clean the main engine ground.

Touch the rubber pieces of all the vacuum lines. Any parts that leave your hand black need to be replaced. There area couple under the airbox at the canister too. I understand you smoke tested and that'* is great for most leaks. Smoke doesn't have much pressure to it.. a vacuum may pull in air that the smoke can't push through the same area.

There'* a good reason why this code was the very first OBD2 code I did a write up on. It'* also been edited several times to add more and more information.

https://www.gmforum.com/obd-ii-trouble-shooting-153/p0171-284036/

Yeah..I remember talking about that with you.

3 months to solve it when it happened to my car. Passing that knowledge on is priceless.
Another couple of folks dealt with it for 4-6 months.. it'* a fun one.

I just added a few more things to check to it again. Need to remember that write up.

How does the EGR tube remove? And (stupid question) how do you get to the other end of the battery cable/the engine ground?

The EGR tube is held to the LIM by a 13mm bolt and to the EGR base by a 13mm nut. It is possible to get this out w/o pulling the TB. However if you haven't cleaned the TB in a while..it'* helpful to have it off.

The ground on your Bonneville is easy to get to by jacking up the car, remove the plastic splash shield under the front of the car and then looking up between the starter and AC compressor. It is held to the engine with a 13mm bolt. On the Impala it is held to the lowest transmission to engine bolt stud with a 15mm nut.

I pulled both the EGR tube and the Auxiliary Air Pump tubes off the engine (EGR tube was a it of a PITA), and they looked new, no pinholes or splits.

Battery'* not that old, cables have no sign of corrosion, when I removed the battery to change the radiator I checked all the connections like I always do (this is the wife'* car, I check all this crap twice 'cause all she knows how to do with a dead car is call me.)

I replaced all the rubber vacuum lines on the engine with new ones, I used hard elbows and T'* in place of the proprietary (you have to buy the whole Evap. line from GM to get one rubber fitting for the TB) crap GM used, the rubber was hard as a rock on all the connectors.

The Evap solenoid and all the other plumbing on the car has never been molested, what I didn't replace is all original on the original ports.

It'* been a full two days of driving now, and (drum roll) still NO codes, stored or MIL. I THOUGHT I had an idea of how these things worked, but I'll be damned if I can figure out why replacing the MAF for a couple of minutes would cure the P0171.

The engine still has a very pronounced miss once in a while, and is not rock steady at idle (which has always been the norm.) The car is 11 years old and has about 105k miles on it, I've decided to shotgun a couple of things to see if they may be causing the driveability problems.

I have ordered a Fuel Pump (fuel gauge has been batty for years, hopefully if nothing else the new assembly will give the car a usable fuel gauge again.) Fuel pressure test was a little under pressure about 50PSI at the regulator.

I also ordered a new Crank Sensor, for no other reason than a hunch.

Thank you guys very much for the help so far.

The code sets once the pcm sees two consecutive igntion cycles where the fuel trims are adding enough fuel to trigger the flag. Takes three consecutive cycles without a trigger to turn the light off on it'* own.

Swapping the maf shouldn't do much except maybe double check the seal on the o-ring. Even that shouldn't be enough of a leak to cause the code. That darn EGR tube is usually a suspect and guilty party. I need to think on this just a little more. There'* something I'm missing.

Quick question--(not trying to hijack your thread bro, just the p0171 drives one crazy enough to rant about it anywhere its brought up...)

If LIM/UIM gaskets were bad, would that cause the code??

I'm not ruling out coincidence. Among the things I've considered while trying to figure this out, is that the fuel pump may be the quiet culprit . I know it'* very, very uncommon for them to fail in this way, but I've seen it happen on other cars over the years where they'll have an intermittent bind or occasional high resistance. Does the GM have a check valve?
How much residual pressure is normal during (a scanner initiated) fuel pressure test? I was using an el-cheapo fuel pressure tester, I have an excellent one, but alas all my fittings are for Bosch and other Euro devices. The pressure went down pretty quick after the pump shut off.
I dismissed it as a probably cause of a lean condition, maybe I shouldn't have? Or is this normal? I should have an adapter for the good gauge today so I can get a reliable number.