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Old 11-14-2018, 09:28 PM   #1
awfulwaffle
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Member#: 409332
Join Date: Dec 2014
Location: SE MI/SE WI
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2013 WRX
PBP

Default Advice please - car idles and cruises rich, fuel trims good

Hi all,

Question regarding an 05 LGT at 155k miles that I'm trying to get set up as a daily driver. Car is road tuned by a local tuner, and only mods are a catless UP and DP.

A bit of background:

When I bought the car, the LTFT C was pegged at 15%, though short term appeared to be handling keeping the car from going lean. Ended up going through it and finding and correcting low fuel pressure and a bad MAF. After replacing the MAF, all my fuel trims are solid +/- 5% at any given time. I've driven the car for ~500 miles since the fix.

My issue/question:

The car's idling and cruising on the freeway at an average AFR of 13.5. Gas mileage is correspondingly low, at 20-22 mpg. I had an LGT of the same year in the past with the same mods, which made the same peak boost, idled/cruised at 14.7 and made 25-27 mpg.

I brought this up with the tuner, and he mentioned that it's typical to see AFRs get into the 13s in cruise and left it at that. To his point, the car doesn't knock under load (except for occasional FBKC during quick shifts).

I took the car through a drive-thru wash to get road salt off of it today, for the first time since I bought it. The wash was toasty, and I saw IATs reach ~75F versus the 30F outside. While in the car wash, the AFRs made their way to 14.7 and stayed there. Driving on the freeway after the wash, the cruise AFR was still 14.7, and I averaged 26.5 mpg over a 20 mile stretch of road (where this car averaged roughly 21 mpg before). Eventually, the AFR pretty abruptly made its way down to the mid to high 13s and stayed there, with fuel economy returning to the previously stated 20-22 mpg.

Pulled and checked the ROM after seeing this, and closed loop target compensation should keep the AFR in the mid 14s during cruise, worst case. So, the car's not hitting it's target AFR at cruise or idle. However, fuel trims are good and actually tend to be on the positive side so it doesn't look like the ECU's trying to pull fuel to compensate.

Would love to hear everyone's thoughts on potential causes. I tend to suspect the AF sensor, though it was replaced recently by the previous owner of the car. It almost makes sense to me that water may have splashed on the sensor during the wash and affected it either electrically or thermally. What I can't wrap my head around is the fact that the fuel trims are good. Am I missing something here?
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Old 11-15-2018, 02:43 PM   #2
awfulwaffle
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Member#: 409332
Join Date: Dec 2014
Location: SE MI/SE WI
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2013 WRX
PBP

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A strong sign from the Subaru deity this morning. Pulling away from stop light, AFR suddenly slammed to 20 and STFT maxed out with LTFT learning its way to max as well, without any change in AFR. Strong gas smell from exhaust, no sign of boost leak. Limped it home, new AF sensor in hand now. Never seen an O2 fail like this, assuming that's what happened.

Last edited by awfulwaffle; 11-15-2018 at 03:52 PM.
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Old 11-17-2018, 06:21 PM   #3
awfulwaffle
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Join Date: Dec 2014
Location: SE MI/SE WI
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2013 WRX
PBP

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Wellp, sensor replaced and car's up and running but still running rich, without any fueling being pulled to compensate. Seems that the sensor failure was a separate issue...

Looking at the ROM, the closed loop compensation at the load and engine speed that I cruise should be between -.235 and -.162.

Just for a sanity check, I took the car through the wash again. Sure enough, my cruise AFRs went back to the 14.4-14.9 range, and gas mileage went up. I think I need to get a better understanding of how various sensors play into the determination of closed loop fuel delivery. I don't know what lives in the engine bay that could be getting wet or somehow affected by water in the wash that would explain this.

Only thing I see in the ROM's closed loop fueling section that I haven't verified to be good in BtSSM is the AF sensor atmospheric pressure correction. Would really appreciate some input from those of you who know closed loop fueling well right about now....

Thanks!
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Old 11-18-2018, 06:34 PM   #4
awfulwaffle
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Join Date: Dec 2014
Location: SE MI/SE WI
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2013 WRX
PBP

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I agreed to let the shop install a DP before tuning, after the car came back with good compression in all cylinders and no boost leaks per the test I originally brought it there for.

Old downstream O2 sensor was seized in OE downpipe, which is apparently also used by 32 BIT ECUs when determining closed loop fueling. The open connector happens to be pointed downwards, directly into the direction of underbody spray when going through a car wash........

Last edited by awfulwaffle; 11-18-2018 at 06:43 PM.
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Old 11-20-2018, 11:04 PM   #5
awfulwaffle
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Member#: 409332
Join Date: Dec 2014
Location: SE MI/SE WI
Vehicle:
2013 WRX
PBP

Default

For anyone who finds this thread in the future - confirmed it was the absent rear O2 causing the issue. Installed a sensor, now fueling targets in cruise and at idle are roughly 14.7.
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