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2014 Buick Regal misfire...or something

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4 years 4 months ago - 4 years 4 months ago #35092 by vaeevictiss
So sorry in advance if this is a bit of a rollercoaster but im not sure where else to look on this thing.

The vehicle is a 2014 buick regal turbo. Its the 2.0L LTG engine found in a bunch of GM vehicles from SUVs to the Camaro and Caddy ATS. Its got 90k miles on it so its starting to get up there. The car was tuned by BNR (sent online) who specialize in this engine back at about 30k and it has run great ever since until it hit this 90ish thousand mile mark.

I started getting a low throttle, low rpm...what feels like a misfire so I started troubleshooting that. Checked all the plug and the center of all were clean but the outer part was a bit sooty. I ended up trying a coil test by buying a new coil and test driving it in each cylinder. The miss was always there and never got any better or worse so i ruled out the coils.

I then decided to hook my tuner up to it and check the vitals. Thats when i noticed the long term fuel trim was over 20%. slightly tapping the gas would drop it down so textbook vac leak. I started looking and spraying brakleen and never got any reaction from the engine. I started listening and finally heard a hiss and traced it to where my catch can plumbed into the intake vac line. I decided to eliminate the whole catch can setup while i figured it out. Still had high fuel trims, still had the miss. Idle through all of this has been smooth as butter.

I did notice on the logs that whenever i would feel the misfire, the knock retard shot way up, sometimes to 4-5%

I then decided to just drop the stock tune back into it and got no change in fuel trims. O2 sensor was fluctuating like it should and staying in the .900s during WOT like it should so i WANT to think there isnt a fuel delivery issue either.

I then figured id change out the sensor trifecta of MAP, MAF, and APPS (no longer TPS on newer cars i guess). This magically dropped my COLD long term fuel trims down to about 10-11%. Now when i tap the throttle in idle the long term rises. Oddly it seems the hotter the engine gets the more the trims seem to rise. Misfire/shutter/hesitation or whatever it is, is still there, but again, only at partial throttle. anything from mid to WOT seems fine and I cant really feel it.

Just did another test drive and after the engine warmed up i would try things like fluttering the throttle and doing a couple WOT pulls and the trim really didnt seem to change much but was just always high (10-15%)

I'm really not sure what else to do other than find a really skilled mechanic. Its not acting like a vac leak any more with the trims rising at idle when tapping the gas. and its not acting like fuel delivery issue with the O2 holding a steady rich voltage at WOT. I even started to wonder if it was tranny slip but it didnt have that typical RPM spike that a slip causes. The RPMs just stay steadily rising as its missing. With the knock retard jumping up, im guessing the knock sensors are good as they are basically doing their job of preventing detonation. the fuel pump pressure is 60ish and the fuel rail pressure fluctuates between 500psi at idle to roughly 3k at WOT.

Im really at a loss but i also included a log in case anything stands out to someone more skilled at reading these things. These were BEFORE i changed out the three sensors though so the fuel trim is a bit higher than it is now but it not a problem to get another log if you think it will help.

Thanks for any help with this.

i.imgur.com/Ae4vKOi.png


EDIT: oh, and there are no codes. even when i saw the long term go over 30% once i didnt get a single code. I think that is what is really making this a real PITA. Not even sure if the knock, miss, and high fuel trim are all even related or totally separate issues.
Last edit: 4 years 4 months ago by vaeevictiss.

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4 years 4 months ago - 4 years 4 months ago #35100 by VegasJAK
You have a lean condition going on... this will give you a misfire. What is the STFT data. Do a graph of RPM, LTFT and STFT and do a snap throttle. Lets see the results(not driving). Not sure what you mean by COLD long term data.

Data that you present indicated a vacuum leak, which you had and fixed. You then changed several sensors and now have what appears to be a fuel delivery problem.

02 voltage/wave go rich by default in WOT so the test is showing that the 02 is working. ST & LT fuel trims that go positive with increase in RPM is indicative of a fuel delivery problem. Too much unmetered air causes the 02 to report a lean condition so the PCM adds fuel, thus the positive trims.

You changed the MAP and MAF sensors and trims got better, but which one did it?? Does your air intake have an IAC. An IAC which is not fully closing or opening too much will cause unmetered air to enter and cause the lean condition. A bad IAC can effect MAP, MAF and TPS data.

Do further checks on the fuel pressure. What does the pressure do when you increase the RPM and at WOT. What reading do you get.

Respectfully, if you cannot resist the urge to change parts, at least do it one at a time and check the results. That way you know which part made the fix and the rest of us can assess the results to help further.

I believe GM's trip codes for 02's around 35 in LTFT.

Look forward to seeing you data...

"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does
Last edit: 4 years 4 months ago by VegasJAK. Reason: Added more info

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3 years 6 months ago #43267 by site80
would have been cool if you had came back with your results. Im experiencing the same exact issue but my car isn't tuned.

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