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Old 05-13-2019, 06:15 PM   #1
numskull50
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Join Date: Nov 2013
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Location: clearwater fl
Vehicle:
2015 FXT Touring
Quartz Blue Pearl

Default 2015 FXT rough idle stalling at stop

I just purchased a CPO FXT touring about a month ago from a local dealer. About a week after my purchase I twice got Code P0172 while driving. I took it to the dealer and they said they couldn't find anything wrong, updated the ECM and blamed the CEL on my insurance's OBD2 plug in. Today after driving home when stopped I got a rough/low idle and the car would stall, No CEL, and started right back up. Any ideas of what could be the cause? Car is stock.
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Old 05-13-2019, 09:39 PM   #2
nine5raptor
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15 WRX Prem. w/ HK
CWP

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Quote:
Originally Posted by numskull50 View Post
I just purchased a CPO FXT touring about a month ago from a local dealer. About a week after my purchase I twice got Code P0172 while driving. I took it to the dealer and they said they couldn't find anything wrong, updated the ECM and blamed the CEL on my insurance's OBD2 plug in. Today after driving home when stopped I got a rough/low idle and the car would stall, No CEL, and started right back up. Any ideas of what could be the cause? Car is stock.
My friend Google says rich condition bank 1. Dirty MAF, bad 02 sensor, bad plug (less likely), or boost leak may be culprits. How many miles? Is this on a single tank of gas?
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Old 05-13-2019, 11:08 PM   #3
numskull50
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Join Date: Nov 2013
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Location: clearwater fl
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2015 FXT Touring
Quartz Blue Pearl

Default

Quote:
Originally Posted by nine5raptor View Post
My friend Google says rich condition bank 1. Dirty MAF, bad 02 sensor, bad plug (less likely), or boost leak may be culprits. How many miles? Is this on a single tank of gas?
73k miles I sprayed the MAF and the code hasn't returned after. Its been multiple tanks of gas. This is my first turbo car, so I don't know how to tell if i have a boost leak or not. I would think 02 sensor would throw a code, which is why I'm at a loss and seeing if this was a common issue with the FXT or the WRX.
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Old 05-13-2019, 11:29 PM   #4
nine5raptor
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Quote:
Originally Posted by numskull50 View Post
73k miles I sprayed the MAF and the code hasn't returned after. Its been multiple tanks of gas. This is my first turbo car, so I don't know how to tell if i have a boost leak or not. I would think 02 sensor would throw a code, which is why I'm at a loss and seeing if this was a common issue with the FXT or the WRX.
I misread and thought the code was with the stall. Rereading, I see it was separate. No experience with it on my wrx, but on other cars, I've experienced stalling when the throttle body is gummed up/dirty. Causes a low idle and the a/c compressor or load from another source will cause the idle to drop enough that it stalls. Could still be a rich condition causing it to stall. I'd change plugs if they were the original (Subaru calls for 60k, maybe? I changed mine at 30k). Spark may be weak.
Also, low or erratic idle can be caused by the oil control valves (part of the active timing system). I had to change one out due to a hunting/rough idle. Failures of that component are common enough. Hard to diagnose though, unless you can see the avcs angles. Dealer will diagnose by moving the valves around. I watched mine on an accessport. $45 later it was fixed.
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Old 05-14-2019, 07:18 AM   #5
numskull50
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Quartz Blue Pearl

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Update: Code P0172 came back this morning along with a rough idle.
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Old 05-14-2019, 09:41 AM   #6
Schoat333
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check the clamps on the charge pipe to inter cooler, and inter cooler to throttle body. I believe there was an issue with them coming loose on the early forester FA20DIT models.
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Old 05-14-2019, 09:47 AM   #7
_brian_
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Crystal White Pearl

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check the battery terminals, mine were loose and the same thing happened
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Old 05-14-2019, 11:54 AM   #8
numskull50
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2015 FXT Touring
Quartz Blue Pearl

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Just got back from dealer they replaced MAF and O2 sensor lets see if that fixes it.
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Old 05-16-2019, 01:45 PM   #9
Mr.Sparkle
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If the issue comes back you may want to check out this thread.

I had the rough idle and stall occurring on my WRX and after following that thread and replacing the part my issue went away.
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Old 05-17-2019, 01:05 PM   #10
enduroxt
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05 obxt

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OP, you should get some kind of software to view your live short term and long term fuel trims. It is hard to guess without knowing what the ECU reports. It sounds to me that your have some boost leak somewhere (that is if you are running rich and MAF/o2 sensors are good). If you are running lean, then it is a vaccum leak.
Not super familiar with your engine design, but there should be a way to find such leaks.

As for the ECU params, I use the Btssm app (see here). Been using this for over 5 years now. Has helped me countless of time. As a matter of fact, helped me troubleshoot poor MAF connection a couple of days ago.
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Old 05-19-2019, 08:02 PM   #11
SubieHondaGuy
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2002 WRX
Silver

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We've done a bunch of bulk head harnesses to fix this concern.
I had one you could watch the short term fuel trim go to -21%, then I could grab the main harness by the VDC unit, wiggle it and it'd jump to +7 instantly.
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Old 05-20-2019, 12:39 AM   #12
enduroxt
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^Yes!

Very similar issue I had on my 06 (OBXT): last week, let the car idle, fuel trim around -6% (which I don't like), opened hood, bent MAF wires close to MAF while watching fuel corrections, and bam fuel trim slowly making it back to 0%.... Very sensitive system.
Now mine is a 15 years old wiring harness while yours (OP) is much newer. So, YMMV.
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