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Old 08-27-2020, 04:56 PM   #1
riksk
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Member#: 518114
Join Date: Aug 2020
Default Mysterius Fueling Issue - Sudden Rich Condition

Alright... since i've asked opinions from various tuners and shops around here and i'm yet to solve the problem, might aswell ask here haha.

Scenario: 2011 WRX, 95 oct conservative tune (no aggressive timing, etc)

Problem:

After any ECU reset, car runs fine for a day or 2, fuel trims looks good, so MAF Scaling and Injectors are properly scaled and tuned. So after a few days letting the car sit, fuel trims would look absolutely wrong. My Wideband would read 9 AFR at a very rough idle, no matter if the car is cold or warm (actually since the engine is so rough it doesnt even get hot before just stalling due too much fuel, as it will just get worse and worse more time it's running).

Fuel Corrections before engine start looks good, but soon as a fire it up.. Fuel Correction and eventually Learning will peg at -25% (trying to remove fuel) just after a few minutes of engine running.

So my first guess was, leaking injector, obviously. But... nope. Injectors are fine.
So coils and plugs might be causing a missfire? Nope, no missfires, no codes, just a rich AFR, new plugs and coils.
Bad MAF? Nope, tested a good working MAF from another car, same problem.

Just by reseting the ECU and restarting the engine, everything goes fine for another day or 2. So it's not a mechanical problem or vac leaks, etc.
Sometimes it happens even when cruising the highway, suddently my 14.7 AFR when cruising will slowly enrich to 13.. 12... until it's no longer possible to drive, so I pull over, reset the ECU, car starts up just fine, like nothing happened.

I already tried to log it, however nothing obvious pops up. It's like, you're crusing with a good AFR, good fuel trims, pretty much no corrections at all (+/- 5% in all MAF ranges), then suddently, BOOM, -25% Correction, and slowly making it's way to Fuel Learning. Reset the ECU and everything comes back to normal, fuel trims settles +/- 5%, car runs good, pulls good with good AFRs.

Additional info:

- Whenver it occurs, the Front O2 reading matches my wideband (up to the factory o2 limits, of course)
- As said, already tested MAF sensor from another car and aswell Coils (even tho I have 4 brand new ones from Subaru)
- Happens randomly, after letting the car sit for a few days, or after cruising for a few hours.
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Old 08-29-2020, 10:07 PM   #2
BURNING4ORCE
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Member#: 356215
Join Date: May 2013
Default

Try starting off with a fresh map/rom and transfer over you tuned calibration. I've had instances where maps are corrupted and flashed into cars and weird things happen(hesitation, etc.). You can also force your car to just run on open loop to see if it does the same thing.
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Old 09-05-2020, 08:39 AM   #3
riksk
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Default

Figured it might be a fuel quality problem. So I will upgrade to E100.
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Old 10-19-2020, 10:29 AM   #4
riksk
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Update in case anyone searches for that:

Solution

Turned out to be the MAF connector and wiring. Had a crack in the ground wire that modified the voltage reading.

Soldered a new connector in the harness, problem solved.

Before, idle voltage was like ~1.40v @ 750rpm, now it is ~1.18v @ 750 rpm.

Also bad fuel did mess up the rest of it as well. So I went full ethanol E100.
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Old 10-22-2020, 12:18 PM   #5
currancchs
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Member#: 488200
Join Date: Jul 2018
Location: Brookline, NH
Vehicle:
2006 WRX TR
World Rally Blue

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Quote:
Originally Posted by riksk View Post
Update in case anyone searches for that:

Solution

Turned out to be the MAF connector and wiring. Had a crack in the ground wire that modified the voltage reading.

Soldered a new connector in the harness, problem solved.

Before, idle voltage was like ~1.40v @ 750rpm, now it is ~1.18v @ 750 rpm.

Also bad fuel did mess up the rest of it as well. So I went full ethanol E100.
Thank you for posting the solution! I really appreciate that you did that.

Just wondering how you found the cracked ground wire, since I think I could have a similar issue. Did you just test the resistance of the ground pin to ground and noticed high resistance or was there something obviously wrong during a visual inspection that tipped you off (or something else entirely)?
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Old 10-23-2020, 06:00 PM   #6
Supraroo
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Member#: 356766
Join Date: May 2013
Location: Las Vegas, NV
Vehicle:
2013 WRX
Ice Silver Metallic

Default

Also had a similar problem, car was running very rich with very low idle and would eventually shut off and threw the P0172 code.

Turned out to be a bad MAF and just carbon build up (my car tends to run a bit rich due to the tune). I had the Subaru dealership do a full engine clean, reset the ECU and fuel trims are within an acceptable range. My car also idles at 750rpm now. It used to idle at 600-700rpm and would sometimes dip to 400rpm almost stalling before the MAF replacement and engine clean.
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Old 10-27-2020, 09:35 PM   #7
riksk
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Member#: 518114
Join Date: Aug 2020
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Quote:
Originally Posted by currancchs View Post
Thank you for posting the solution! I really appreciate that you did that.

Just wondering how you found the cracked ground wire, since I think I could have a similar issue. Did you just test the resistance of the ground pin to ground and noticed high resistance or was there something obviously wrong during a visual inspection that tipped you off (or something else entirely)?

I had my computer hooked up while the engine was running. I was live logging the MAF Voltage while touching the MAF wiring until the engine died when I messed around the connector. So I removed the wire tubbing and found one cracked bare wire.
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