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Discussion Starter · #1 ·
I have a completely stock 2012 2.5L MTX Outback with about 200k miles. I have had a P0171 code that shows up when idling for longer periods (more than 3 mins). I did recently replace the MAF, Front O2 sensor, and PCV since I am around 100k since last replaced and it was the first suggestions when referencing this code. I borrowed a buddy's scanner to look at A/F values but I do not know how to evaluate what is going on and how to deduct what to look at next. Can anyone walk me through what values I need to look at and what to do next with the vehicle?
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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Have you seen the other posts for P0171 on this forum?

Does your friend's scan tool save data logs?

There is a lot to look at for correlations in engine management and it's not just the AF values. The AF on yours should be steady at about 2.2V, or if the tool shows Lambda, between .998 and 1.1 Lambda. You can have this value all day long, but you have to compare it to the fuel trim states in relation to rpm, tps, ect, air temp, knock correction, the rear O2, MAF and MAP. I cover a lot of the parameters in my P0420 Diagnosis thread because engine performance is also related to that code.

Have you checked for vacuum leaks?
Any other codes besides this one?
What brand fuel are you using?
How old is the battery? Are the cables in good shape?
 

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I would look for a vacuum leak.

An idling engine creates a very high vacuum. (more likely to suck air thru a leaking hose, seal, gasket) The computer responds by increasing SHORT TERM fuel trim.

The 'typical' way to test for vacuum leak in the intake plenum is to use a 'smoke generator' unit to pump into the plenum and look for leaks.

There are several VERY good uTube videos showing this procedure in detail. (I recommend "South Main Auto" channel, this guy is a master tech.)
 

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Discussion Starter · #4 ·
Thanks for the direction. Originally I looked at all the vacuum hoses and made sure the intake pipe was assembled correctly. The car ran good but would randomly stumble at idle so for some reason and I was thinking a possible intermittent wiring issue. After using the scanner and seeing the MAF and O2 values were steady and reading from the replies here to check vacuum leaks I decided I needed to do exactly that. So I did a deeper dive looking for vacuum leaks and found a leak at the intake to head on the forward most cylinder (Cylinder #1?) The gasket was stiff and cracked with a section of it falling out when removing the intake. I was surprised at how bad the condition the gasket was in. Thanks again for the help. I was just too pig headed to look at the obvious I guess?

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