I had a similar issue on mine. Maxed out short term fuel trims at idle, pretty normal trims driving around. Chased it for a while, dealer wanted to throw hundreds of dollars of parts at it. Turned out to be a rusted out brake booster. Easy to check by popping off the hose to the booster and plugging it with your finger while looking at the short term trims at idle.

If trims lower as the engine goes under increasing load it is almost definitely a vac leak somewhere.

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will do, I grabbed some new gaskets and TGVs (deletes). Installing mid next week. With the manifold off it's much easier to see everything. The smoke tests definitely narrow the issue down to the passenger TGV axle shaft housing thing, or it's the PCV system, don't really see anything else in that area that'd be under vacuum. I learned I can mount the old TGV motors back on, connect 'em and they'll still "work", that way I won't have a CEL. Hmm, anyone got a copy of Cobb's tuning software..? Are people using ECUFlash or w/e to opensource tune their WRX's? I'm generally new to self-tuning, but would love to get into it somehow. Thanks guys!

Update on my issue and confirmation on keane12 issue as well as a few others. I tested the vacuum on my brake booster and sure enough, big vacuum leak. This can easily get overlooked, since my brake booster visually looks brand new and no braking issues at all. This can also be missed in a vacuum system smoke test, as mine was leaking at the pedal shaft and never saw smoke in the engine bay. Anyone having similar issues, save yourself a significant amount of money in new OEM sensors and check the brake booster and chassis to engine grounds. Our cars are notorious for weak grounding and can cause sensors to act up.

Dp==d

Hey thanks for the tip about the brake booster. I unplugged the vacuum hose from the brake booster side while leaving it still connected to the intake and plugged it, car still ran with ~17.5 AFR while idle at operating temps. Gotta be a leak somewhere, right!? Also, thanks for the ground wire tips, I found 4 of them, on top of trans, and then a few on the sides. The ground strap I have looks like it had a coating but is dirty af. The connections were all cleaned up with a wire brush. Same thing with battery terminals. Shined em up cuz they needed it lol.. Car still running with ~17.5 AFR idle at operating temps. And, FWIW, the car _does_ run better than it did before, so some improvement has been made. Once I revved it a bit, the AFR started to creep more up to around ~18.5 - 20 at idle. I guess next up is I'm gonna look at the up-pipe and cross pipe. I have replacements of both and all the gaskets needed. So I'll have a look under the car. Already found a big crack in the heat shield of the up-pipe, but I'm not hearing an obvious leak with the car running while I'm under it.. idk, this has been a fun journey tho, really getting to know my car through and through. Basically the issue is once the car hits closed loop, AF Correction 1 (real time fuel trim) goes to +25%, that's right positive 25 percent, indicating the car is running lean. And it stays at +25%. And then the AFR is always 17% or higher. Only in closed loop do I have this issue. I don't have much else to replace, so I guess I'm really narrowing it down, brute force style lol. I replaced my boost controller and one of the purge valves, I ordered a replacement for the other one that I've yet to replace.

What's been done (my brute force troubleshooting method XD):
- new MAF sensor
- Replaced front O2 (AFR) sensor (was probably 10 years old maybe, car is 2013)
- Replaced downstream O2 sensor (seriously was charred af looking, I have an oil leaking in the intake manifold issue or some ****.. I think I need an AOS?)
- TGVs replaced with deletes (did find a leak in the passenger side one where the axle is)
- TGV <---> engine block gaskets
- TGV <---> intake manifold gaskets
- new o-rings in fuel injectors
- new turbo inlet from Perrin
- new vacuum lines where needed (my wastegate tee lines were pretty cracked and nasty)
- new hose clamps pretty much everywhere

What I have left to do:
- Replace up-pipe with one from Cobb
- Replace cross pipe with one from Grimmspeed
- using all new gaskets for the above work
- Replace stock intercooler with aftermarket ebay one (looks way tf better ngl)

I had some dumb issues along the way, like when I reinstalled the intake manifold, I stripped the threads in one of the TGVs and the fuel injectors leaked like crazy. Fixed all of that.. I swapped vacuum lines on both the purge control solenoid valves and ran the car, so thought maybe I ****ed em up. Just gotta replace the other one. Didn't seem to make any difference to how the car runs tho.

It is neat to see the AFR jump up tho.. I had the BPV vacuum line unplugged mistakenly with car running, noticed AFR of 20.33%. Found the mistake and plugged the line, then the AFR stays at just over 17%. So, the system seems to kinda work, just gotta still be a leak, somewhere, right? Anyway, thanks for coming along for the ride guys and thanks to all of your contributions. Called my tuner... they have not called me back and it's been a week.. rolling my sleeves back up I guess haha.

I took it to my tuner and they mentioned fuel pressure is 10 psi lower than it should be at idle. Looks like that was probably the issue, will update as I learn more.

The fuel pressure being lower like that at idle was the issue. Thanks guys

So what parts were changed?

Did you figure out what was causing low fuel pressure at idle?

It must some type of vaccum issue. It’ll only pull fuel like that when it’s trying to correct something try to make a home made smoke machine and smog out the system. It’ll atleast be a good start