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Lean Mix In Cylinder Bank 2. What to check first?

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2017 GT with inferred flex-fuel tune. Mods listed below.

I was entered in a two-day autocross last weekend. I had just put almost a full tank of E85 in the tank. Before that, the little remaining fuel was almost all E85 but from a different station. I did not test either fuel.

On Saturday while queued up to launch, idling, the rpms rose to about 1500 and hovered there for 8-10 seconds, then went back down to normal idle. I ran the car with no issues. The same thing happened a couple runs later. Car continued to run fine and I didn’t notice that or any other issue the rest of the day.

The next morning at the hotel, the car started and the CEL was on. Code P0174….lean mix in cylinder bank 2. I drove the car nice and easy over to the event. There is a Ford mechanic in my club and I asked him what he thought. Said not to run the car if I was feeling power loss or anything strange, but if all seemed well, he’d run it. None of that was happening, so we dumped the code and I ran it. It ran well enough to get 2nd overall in Raw Time, and no issues.

Drove well over a hundred miles home, have restarted the car and run it a couple times. Still no apparent issues and CEL remains off. Not sure how many cycles/miles before the light may come back on. My local mechanic will be checking the car tomorrow. He’s not a Ford guy, nor the guy who did the mod and tunes, but I trust him and have him handle the simpler stuff. What guidance can I give him to diagnose? Start with checking upstream O2 sensor and try cleaning the mass airflow sensor? Thanks

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Any codes to do with running lean means DO NOT RACE THE CAR. Dumping codes does not make the problem go away. Its a warning of impending doom, so I would strongly advise a log ASAP.
Ouch. I hear you. Not sure why the Ford tech would say to run it. And yes, I’m aware that dumping the code doesn’t fix anything - As I wrote above, I’m expecting CEL to come back on. Should’ve erred on the side of caution.
 
If I can recall you had weird stuff going on back in the fall - goofy engine sounds and codes. Who knows what was going on. Was it pre-ignition. Maybe the new code is a confirmation. I'd call your tuner and see what he thinks about the new code.
Good memory…no weird sounds, but a cylinder 5 misfire code right off the bat last fall. They sorted that out by adjusting the tune. I did talk to the tuner before posting. He said it could be the tune, but not likely. Thinks it’s mechanical, a bad sensor, air leak etc.

I see now it was you who commented last fall that the headers and E85 combo aren’t friendly to O2 sensors. And it was pretty cold out the morning it threw the code.
 
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Someone needs to school me on this, whenever I get a lean bank code, it's almost always an O2 sensor, but the single cylinder code is usually an injector. How does the ECM decide which cylinder it is? How many sensors per cylinder, does the car have?
Dies it decide which one through plug voltage or some other means?
 
How does the ECM decide which cylinder it is?
Ford IDS has a Power Balance tool that I believe uses input from the crank speed sensor (since each cylinder will have a dedicated portion of trigger wheel angle) to determine what cylinders are performing well and which are underperforming compared to the rest.
The below example shows a cylinder 4 misfire. I'm assuming the engine management software uses the same method and can tell if it doesn't get a combustion event during that cylinder's portion of the trigger wheel sweep (deceleration, even if ever so slight) and probably sets a flag after it occurs a certain number of times.
PowerBalance.PNG
 
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Got the log done and it showed no issues whatsoever. Running nicely, at least for now. Tech recommended I put the car through a couple more drive cycles to see if CEL comes back on.

Would an O2 sensor possibly fail intermittently, not always throwing a code? Not sure what I should do next.
 
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Got the log done and it showed no issues whatsoever. Running nicely, at least for now. Tech recommended I put the car through a couple more drive cycles to see if CEL comes back on.

Would an O2 sensor possibly fail intermittently, not always throwing a code? Not sure what I should do next.
Yes they will, my 2010 F150 tossed a check engine light, then reset itself when I went to restart it. A couple of month's later it came back on... O2 sensor. Replaced all 4 with Bosch parts ( no Ford dealers around here) it still tossed an O2 code... WTH? Pulled the up stream, brand new Bosch sensor out, replaced it with another, it cleared.
So yes, they can do it intermittently
 
Yes they will, my 2010 F150 tossed a check engine light, then reset itself when I went to restart it. A couple of month's later it came back on... O2 sensor. Replaced all 4 with Bosch parts ( no Ford dealers around here) it still tossed an O2 code... WTH? Pulled the up stream, brand new Bosch sensor out, replaced it with another, it cleared.
So yes, they can do it intermittently
Thanks. Glad that wasn’t a dumb question. Hah.

The car’s now gone 170 miles and 5 drive cycles since resetting the CEL. It still hasn’t come back on, so between that and the clean log, I’m going to run her, but pay close attention.
 
81
93
Exp. Type
W2W Racing
Exp. Level
Under 3 Years
Scottsdale
I'm running into strange issues too when I am tracking my car. I pulled the codes and had multiple bank misfires and ABS codes. It was an intermittent hesitation when I was accelerating. This weekend I was out at the new Attesa Race Track. The car starting have the hesitation and progressively got worse as the sessions went on. Then car went into limp mode. I pulled codes and only ABS codes. Figured out it was a ABS sensor causing the misfire. Check your wheel bearings and sensors!
 

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