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Discussion Starter #1 (Edited)
2008 STI, No Mods - All Stock, 47k Miles. Idle fluctuates a little.

Went to fill up tank a few weeks ago. Shut car off, filled tank, started car and it rumbled a little violently. I figured maybe an injector. Shut car off and restarted and it ran well with no issues. Drove 40 miles and all seemed fine. Looked for parking and CEL went on. Read code P0171.

I figured maybe injectors were dirty and added Techron Complete Fuel System Cleaner for next fuel fill. Cycled through that tank and CEL popped back on.

Figured it might be a dirty MAF sensor, so I pulled the sensor and it was dirty. I cleaned with CRC MAF sensor cleaner. The lowest reading for the MAF is 2.39g/s @ 1.1V @987rpm and highest reading is 252.06g/s @ 4.38V @ 5672rpm.

Few miles later CEL came on again. I figured maybe the front A/F sensor might be shot because of age of car. Changed front A/F sensor and a few miles later the CEL came on again with same code.

Checked intake for vacuum leaks with smoke and no leaks.

My exhaust seems in good shape but have not checked to see if I have a leak. I don't think the exhaust is compromised in any way, but all speculation.

I am logging in romraider but it has been such a long time since I did any logging or tuning on an ecu that I have forgotten what to look for in order to pinpoint the problem. I know you want to look at STFT and LTFT but not sure what else to look for. I also downloaded Learning View but have yet to try it on the car.

Any suggestions? Any key points I am missing or need to consider? Any help would be great.
 

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I had the same code
Try swapping the front o2 (Upstream)
I cleaned the MAF and ended up working for me
Bought a secondary one just to be ready in case cleaning didn't work
Heres my thread below as reference. It lists the readings off the AP to help give you an idea.
I was popping the CEL when i was WOT but after some troubleshooting the CEL never came back but the readings were still off so continued to troubleshoot till everything was normal readings

Lemon to 400+ HP Heaven
 

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Discussion Starter #3
I had the same code
Try swapping the front o2 (Upstream)
I cleaned the MAF and ended up working for me
Bought a secondary one just to be ready in case cleaning didn't work
Heres my thread below as reference. It lists the readings off the AP to help give you an idea.
I was popping the CEL when i was WOT but after some troubleshooting the CEL never came back but the readings were still off so continued to troubleshoot till everything was normal readings

Lemon to 400+ HP Heaven
Changed out the front O2 last week and CEL appeared again.
 

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Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
Inspect all vacuum and PCV hoses, replace/repair as required
Inspect all hoses and connections in the air intake system
Inspect and/or test the intake manifold gaskets for leakage
Check for a dirty fuel filter and proper fuel pressure
Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool
If you have access, you may want to run a smoke test

check your intake system...possibly torn at the turbo...hopefully no fuel pump issues but its still not hard to access...
 

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Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
Inspect all vacuum and PCV hoses, replace/repair as required
Inspect all hoses and connections in the air intake system
Inspect and/or test the intake manifold gaskets for leakage
Check for a dirty fuel filter and proper fuel pressure
Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool
If you have access, you may want to run a smoke test

check your intake system...possibly torn at the turbo...hopefully no fuel pump issues but its still not hard to access...
Im not sure about using brake cleaner, I would use the specific MAF cleaner spray.

OP. If cleaning it for the second time doesn't do the trick. Perhaps try swapping it since I hear that it's extremely sensitive.
 

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check your intake system...possibly torn at the turbo...hopefully no fuel pump issues but its still not hard to access...
yeah i would visually check all the lines for any cracks/tears - despite a smoke test coming up negative. 2008, some of the hoses gonna be hard/stiff/out of shape.
 

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Discussion Starter #7
MAF has been cleaned. I looked over the intake, all vacuum lines, and it is all in good standing condition.

Since I had that violent rumble with the initial startup it might be a gummed up fuel injector. I am going to have the injectors cleaned out this weekend.

I will keep you posted on my results.
 

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Discussion Starter #8
I was told my intake manifold gasket was bad b/c my tumbler was shot. I am assuming it failed closed.

All was repaired and STi is still throwing a P0171 code. Dealership said they pressurized system after the TGV and gaskets were replaced and the brake booster is bad.
 

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I was told my intake manifold gasket was bad b/c my tumbler was shot. I am assuming it failed closed.

All was repaired and STi is still throwing a P0171 code. Dealership said they pressurized system after the TGV and gaskets were replaced and the brake booster is bad.
Hows everything now?
 

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Discussion Starter #10
Brake booster was bad and I replaced it and the CEL disappeared.

I cut the brake booster open and the bottom half had corroded (don’t pay attention to the position of the snorkel for the vacuum line). Looks like it was sucking moisture in for a little while.




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