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Discussion Starter · #1 ·
I got a P0170 code on my 2012 Nissan Altima SL and I wanted to try and track down a vacuum leak. Does anyone have or know where I can get a chart of the different vacuum connections on the 2012.
 

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I presume you mean P0171, system lean. The most likely culprit for that is actually a dirty MAF, although there's a whole list of other things including vacuum leaks, exhaust leaks and a bad A/F (front O2) sensor. There isn't really a blanket hose routing diagram in the ESM, much of it is system-by-system, but the quickest way to rule a vacuum leak in or out is by watching short term fuel trim (STFT, or A/F-alpha on a Nissan scanner). With the engine warm, if STFT goes significantly positive at idle but comes back down around zero at 2K RPM, there's a leak. If you don't see much change between idle and 2K, then the problem is something else.
 

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Discussion Starter · #3 ·
Thank you for this. I did mean the P0171. I cleaned the MAF and reset the code. I just drove it for about 15 minutes and the code has remained off so far.
thanks again.
 

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Discussion Starter · #4 ·
I presume you mean P0171, system lean. The most likely culprit for that is actually a dirty MAF, although there's a whole list of other things including vacuum leaks, exhaust leaks and a bad A/F (front O2) sensor. There isn't really a blanket hose routing diagram in the ESM, much of it is system-by-system, but the quickest way to rule a vacuum leak in or out is by watching short term fuel trim (STFT, or A/F-alpha on a Nissan scanner). With the engine warm, if STFT goes significantly positive at idle but comes back down around zero at 2K RPM, there's a leak. If you don't see much change between idle and 2K, then the problem is something else.
alright, so the check engine light came back on and I”m getting the same code, P0171.

I’m not sure if I am equipped to do watch STFT although I do have an Innova 300 RS OBD scanner which says it can provide live stream data. Trying to figure that out.

what would you do next?
 

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alright, so the check engine light came back on and I”m getting the same code, P0171.

I’m not sure if I am equipped to do watch STFT although I do have an Innova 300 RS OBD scanner which says it can provide live stream data. Trying to figure that out.

what would you do next?
An easy way to check for a vacuum leak is to attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
 

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Discussion Starter · #6 ·
Unfortunately, you lost me at “Full vacuum source”. I do have a mightyvac MV8000 which says it will perform vacuum leak test. I checked the bellows and connections and they all appear to be good.
 

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Discussion Starter · #7 ·
I presume you mean P0171, system lean. The most likely culprit for that is actually a dirty MAF, although there's a whole list of other things including vacuum leaks, exhaust leaks and a bad A/F (front O2) sensor. There isn't really a blanket hose routing diagram in the ESM, much of it is system-by-system, but the quickest way to rule a vacuum leak in or out is by watching short term fuel trim (STFT, or A/F-alpha on a Nissan scanner). With the engine warm, if STFT goes significantly positive at idle but comes back down around zero at 2K RPM, there's a leak. If you don't see much change between idle and 2K, then the problem is something else.
so, I was able to look at the STFT on my scanner. At idle it was very close to zero and then when I went to 2000 rpm it dropped to around -10.
 

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I was able to look at the STFT on my scanner. At idle it was very close to zero and then when I went to 2000 rpm it dropped to around -10.
It isn't vacuum causing the problem, then. It would be way rich at idle, zero is about where it should be.
 

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The most likely cause at this point is a MAF issue. Bad A/F sensors will generally make fuel trims read high across the board, so that can be pretty much excluded. I'd try recalibrating the MAF first (IAVL procedure, your scanner probably has it in work support). Stream the MAF reading at a steady RPM before doing the IAVL, then perform it and re-check the reading afterward at the same steady RPM. If there's a big change in reading before and after, chances are the problem was just an aging MAF and the IAVL fixed it. I've seen them drift as much as 30% as the components age, but it doesn't mean the MAF is bad, just that the ECM isn't on the same page with it anymore. IAVL fixes that. If you don't see much change before and after, I'd say try replacing the MAF.
 
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