2000 Monte Carlo 3.4L
The car bounces all around from 500-800 rpm when sitting at idle. When you're moving, the car runs smooth but at a stop with no throttle it starts to fall on it's face.
The only code being thrown is P0171.

Cleaned the MAF sensor and saw no difference. If I start the car and run it with the MAF unplugged, it idles smoothly but at ~1500, which it would do just fine if I held the throttle there while it's plugged in...so not much to learn from that.
Spark plugs are all firing, sprayed starter fluid all over different places and saw no evidence of a vacuum leak. Might borrow someone's known good MAF and see what I get.

Any suggestions?

 

IMO, I think it does tell you something. The car shouldn't be idling anywhere near that high with the MAF unplugged. But the fueling strategy is different without the MAF which points to a strong possibility of cause.

With the MAF plugged in, it only gives as much fuel as it measures air going by the sensor. If air gets in after the MAF, it receives no matching fuel. This causes it to go lean (hence your code) and stumble (your idle issue as it adds IAC to try to keep the engine alive) if enough additional air is getting in downstream.

With the MAF unplugged, the engine doesn't care where the air is getting in at. It fuels based off of pressure in the manifold. That includes air coming in from the IAC, but also include air leaks from several other potential sources. Because its getting air elsewhere (and is now receiving matching fuel for that air), it is driving the idle up substantially. Id bet the IAC values are showing the valve totally shut as the engine tries to close off external air to bring the idle down - but it can't because the post TB leak is letting in so much air on its own.

The reason it cleans up at higher rpm is because the leak is a fixed size. That size is apparently large relative to idle airflow (especially if its doubling idle rpm with the maf unplugged), but shrinks in proportion as rpm goes up. Opening the throttle blade to accelerate also decreases the amount of vacuum in thr manifold as it now has an easy path to get air from - so that further reduces the effect of the leak. Id bet you'd still see the leak show up in fuel trims / O2s, its just not big enough to cause driveability issues at normal driving rpm.


So short story from the very limited data set we have here, I'd put my money on a vacuum leak.

 

fuel pressure regulator ?