Idle goes rough randomly. Stock tune..

Discussion in 'Issues, Repairs, Warranty, TSB, Recalls' started by Andy13186, Dec 21, 2019.

  1. Locksmith

    Locksmith Member

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    Our 2017 GT started doing this about 6 months ago and got really bad within the last month. 54k miles, no codes and all stock except Magnaflow mufflers. New plugs, air filter and cleaning the throttle body didn't help. Actually seemed to make it worse. I have the factory CD manual and the list of things to check for this symptom is long, pretty much everything. I'm fortunate to have a mechanic two doors down from me who has an Autel and he suggested a throttle re-learn. We did that Wednesday. The very first drive home, 17 miles, was actually worse but Thursday morning it was fine. Has been ever since.

    My question, I guess, is why would this not throw a code? With all the sophisticated engine management, I don't understand why it wouldn't know somethings wrong if it stalls out.
     
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    Andy13186

    Andy13186 Well-Known Member

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    Mines been 100% since the VCT solenoids. My only codes were actually for the front o2 sensors on both sides, replacing them didnt fix the issue. I too wish the computer would identify this problem and throw a code for it but it never did for me, it was frustrating.
     
  3. Locksmith

    Locksmith Member

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    Yeah, very frustrating. Ours stumbled just a bit on the way home from an evening out. Similar to a misfire for just a half second. Happened right after a stretch of 75mph interstate at the first complete stop. That's where it all started months ago.

    The next thing my mechanic neighbor wants to try is hook up both an analog fuel pressure gauge and his Autel. Take it for a drive and monitor live data and the gauge simultaneously. He's thinking the computer may be reading something different than what's happening in real life, maybe causing a rich or lean mix. We'll see.
     
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    Andy13186

    Andy13186 Well-Known Member

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    heres the video I made about it if you are interested
     
  5. Diocletian

    Diocletian Well-Known Member

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    My idle, rough start, misfire issues were caused by a vacuum leak, stock airbox had a small crack in it just enough to screw with the MAF. Replaced under warranty, all issues gone
     
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    Andy13186

    Andy13186 Well-Known Member

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    Interesting, where on the airbox was the crack?
     
  7. Diocletian

    Diocletian Well-Known Member

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    Towards the bottom, back left corner of the airbox
     
  8. Mrpopper239

    Mrpopper239 Member

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    So I am going to replace my solenoids as well, but I’m wondering can I re use the valve cover gasket? Or do I just remove the valve cover and put it back on without it since it’s probably adhesived on
     
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    Andy13186

    Andy13186 Well-Known Member

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    At the end of my video with the dealer print out, it looks like the dealer replaced both the gaskets, may as well put new ones in there while you are in there in my opinion since they are only 9$. JL3Z6584C and JL3Z6584D gaskets were replaced by my dealer on this fix.
     
  10. Dana Pants

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    I haven't caught the stumble/stall failure on datalogger yet, but have been able to use Forscan to data log VCT angle vs target behavior and its pretty ugly. The car seems to idle worse when the angles are lower.

    Could someone with a "Good running car" plot VCT angles vs time and someone with a "bad running car" do the same for something like idling at a stoplight?

    I looked at the service manual and to take off the valve covers of a 2018 5.0 looks ugly enough that I'd rather not do it for no reason. VCT_Targets_Vs_Actual_20200415.png
     
  11. geep81

    geep81 Well-Known Member

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    If it makes you feel any better my 2019 GT did this and my 2020 GT does it too. This "flutter" noise. Weird as it is it seems normal.
     
  12. Dana Pants

    Dana Pants Well-Known Member

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    So I did more data logging yesterday and the car only misbehaved really once with a rough idle/near stall. As stupid as it is, it looks like that when the RPMs are winding down on the way to a traffic light the car goes into closed-loop idle control too soon, closes the throttle plate due to high engine RPM, and is surprised the car nearly stalls. VVT performance seems to play no part in my problem at least. If this was an older car I would adjust the throttle plate set screw to raise the minimum airflow. I'm going to take a peek at the throttle plate later today and see if I get any inspiration. 20200418_RPM_vs_Ignition_Advance.png 20200418_RPM_vs_Throttle_Position.png 20200418_RPM_vs_VVT.png
     
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  13. Dana Pants

    Dana Pants Well-Known Member

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    Throttle body was pretty schmutzy. TBD if cleaning helps 443F7237-6E20-4F4D-AE40-D48249B16F57.jpeg 199E905A-F641-48FA-954F-5B30F9CA0C4D.jpeg
     
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  14. Dana Pants

    Dana Pants Well-Known Member

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    So the idle waveform and behavior definitely changed after cleaning the throttle body. Motor spends more time "over" the RPM target and less time under it struggling & shaking. TBD if random stall is really solved, but definitely a step in the right direction. Idle side by side after throttle body cleaning.png
     
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  15. fahadco80

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    Thanks for letting me know :)

    Actually my car at the dealer doing TSB 19-2140, they told me this is normal but I insisted to do it. Will receive the car tomorrow Saturday, will keep you updated, wish me luck :)

    Regards

    Fahad
     

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