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Old 06-25-2019, 07:26 AM   #1
numskull50
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Default 2015 FXT Running rich Stalling at Idle

I just purchased a 2015 CPO FXT touring a few months ago from a local dealer with 73K miles. 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. 500-600 miles later, same issue and the same dealer replaced the O2 sensor. its been about 1000 miles since then and code popped back up again. I have a cheap ELM reader and torque and read LTFT on -45 and STFT +35 at idle in gear when warmed up. Car seems ok when driving at highway speeds, but at stop lights idles rough and occasionally stalls. I've spray cleaned the MAF and MAP sensor was covered in oil when I checked it. Any idea whats to look for next or to direct the dealer to try?
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Old 06-25-2019, 10:30 AM   #2
X49
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The MAF and MAP should not affect fueling at idle (except within a minute or so of a cold start). The next things I would rule out are a bad rear O2 sensor (assuming they've replaced the front), or a leaky fuel injector.
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Old 06-25-2019, 10:51 AM   #3
numskull50
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Quote:
Originally Posted by X49 View Post
The MAF and MAP should not affect fueling at idle (except within a minute or so of a cold start). The next things I would rule out are a bad rear O2 sensor (assuming they've replaced the front), or a leaky fuel injector.
They did replace the front O2 sensor. Whats got me is that fuel trims when driving appear to be around 0 its only at idle when the issue occurs.
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Old 06-25-2019, 01:22 PM   #4
nine5raptor
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What's your AFR at idle? Bouncing around 14.7? If so, then the fuel trims are doing their job and the combined -10 is getting it back to where it needs to be. Not sure why LTFT would be stuck at -45 tho.

One option would be to disconnect the battery to reset the fuel trims. Then start the car and let it idle up to temp. Turn the car off and restart (this ensures your front O2 and AVCS are active). Then check fuel trims and AFR.

Intermittent Idle issues can also be caused by a bad Oil Control Valve (part of the AVCS). I had to replace 1 OCV so far on my FA20DIT because of rough idle, near stalling.
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Old 06-25-2019, 01:32 PM   #5
numskull50
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Quote:
Originally Posted by nine5raptor View Post
What's your AFR at idle? Bouncing around 14.7? If so, then the fuel trims are doing their job and the combined -10 is getting it back to where it needs to be. Not sure why LTFT would be stuck at -45 tho.

One option would be to disconnect the battery to reset the fuel trims. Then start the car and let it idle up to temp. Turn the car off and restart (this ensures your front O2 and AVCS are active). Then check fuel trims and AFR.

Intermittent Idle issues can also be caused by a bad Oil Control Valve (part of the AVCS). I had to replace 1 OCV so far on my FA20DIT because of rough idle, near stalling.
AFR is around 14 at Idle, just the fuel trims going crazy. Every 500 miles or so I get a check engine light and need to reset the fuel trims. I'll look into the Oil control valve.
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Old 06-25-2019, 09:03 PM   #6
CaptainSlowbaru
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Quote:
Originally Posted by X49 View Post
The MAF and MAP should not affect fueling at idle (except within a minute or so of a cold start). The next things I would rule out are a bad rear O2 sensor (assuming they've replaced the front), or a leaky fuel injector.
This statement is completely wrong.

The MAF and MAP sensors' entire job is to tell the ECM how much air is entering the engine for... wait for it... proper fuel mixture!

Furthermore, the rear O2 sensor is there to monitor the cat- nothing more. No fuel control whatsoever.

If the dealer can't find a problem when the fuel trims are maxed out, FIND A NEW DEALER- they obviously don't have a freaking clue, and don't deserve your (or anyone's) business.

Last edited by CaptainSlowbaru; 06-25-2019 at 09:13 PM.
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Old 06-25-2019, 10:31 PM   #7
numskull50
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Pulled neg battery cable on lunch and reset the fuel trims. Drive home was around 0 LTFT. Im not really sure what happened that maxed it out before. Thinking I may have a vacuum/boost leak somewhere. Its just weird I can time the CEL to 5-600 miles. My drive home was all bumper to bumper so didnt really get into boost.
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Old 06-28-2019, 10:51 PM   #8
X49
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Quote:
Originally Posted by CaptainSlowbaru View Post
This statement is completely wrong.

The MAF and MAP sensors' entire job is to tell the ECM how much air is entering the engine for... wait for it... proper fuel mixture!
The MAF is used for open loop control, which only occurs at high loads, or at before the O2 sensor warms up. In closed loop, such as at warm idle, MAF calibration doesn't matter.
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Old 06-29-2019, 12:41 PM   #9
SubieHondaGuy
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Bulk head harness has fixed a bunch of p0172 issues on the turbo foresters.
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Old 06-29-2019, 10:44 PM   #10
CaptainSlowbaru
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Quote:
Originally Posted by X49 View Post
In closed loop, such as at warm idle, MAF calibration doesn't matter.
So you mean to tell me that on a warm engine, you could unplug the MAF, or have a dirty MAF, and the engine will run completely normal with fuel trims at 0?



Yeah, ok.
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