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Chasing an intermittent rough idle and slight load misfire

Discussion in 'SN-95 and New Edge Mustangs' started by 99GTMichael, Jul 27, 2019.

  1. 99GTMichael

    99GTMichael New Member Established Member

    Messages:
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    Location:
    Missouri
    Hello Folks,

    I'm chasing a rough, uneven sounding idle and an intermittent slight misfire under load on my '99 GT T-45 with 99,xxx miles.

    Little bit of service history to start...

    Autolite 104 spark plugs May 2011 80,800 miles

    Motorcraft fuel filter 2016 94,429 miles

    8 Motorcraft ignitions coils September 2017 97,9xx miles (moisture identified in cylinder 5 spark plug tube and coolant in cylinder 8 spark plug tube)

    Motorcraft intake manifold gaskets and all upper and lower Fel-Pro fuel injector o-rings June 2018 98,9xx miles (I cracked the "plastic base" of the #5 injector, but reinstalled it because I didn't think it would be an issue when held down by the rail)

    8 Motorcraft SP493 spark plugs and 8 Motorcraft WR-6128 coil-boots July 2019 99,45x miles (I believe I found some carbon tracking on the spark plug porcelain and there was definitely some rust on the hex)

    Drive-train of this GT is completely stock minus a SLP LM1 Catback exhaust.

    I'm hearing what I would describe as a burble/pop from the exhaust that sounds like an engine miss. You can feel in the vehicle some unevenness to the idle.

    Driving down the road at about 40-45 MPH in 4th gear, I can hold the throttle just right to cause an engine miss that feels like the pretty typical Ford COP misfire. I can also recreate this at about 65 in 5th.

    Of course... no service engine light or misfire codes.

    After installing the coil boots and spark plugs last night, I test drove the Mustang back to work because I forgot my paycheck. I noticed immediately the idle note was not improved as I would have liked. The car runs out down the road really well.

    Once getting to work, I used the Modus Edge to try to pull some codes and check misfire data. The only stored codes were a P1000 (battery was dead from sitting in the garage) and a P0153 (H02S Bank 2 Sensor 1 Circuit Slow Response... evidently not requesting SES).

    I was pleased to see a P-code because I have a direction to follow now. I first check misfire data pids and see that I have 163 cylinder 5 misfires counted. (Note previously that cyl. 5 has the cracked injector base...). I did not see this change before leaving work.

    Moving on to emissions data pids, I see 02S21 drop to around 100 mV and not switch... 02S22 goes to 900 mV accordingly. Very interesting!

    02S21 quickly comes back to switching and 02S22 begins to drop slightly. Looking to short term fuel trim bank 2, the scanner graph shows dead rich at 33% while 02S21 wasn't switching.

    I'm inclined to diagnose a bad 02 due to the fact that it comes in and out of switching properly. To my mind, I believe the other sensors and pids are reacting properly to what O2S21 is showing. O2S21 goes to low voltage, O2S22 goes to high voltage, short term fuel goes way positive and long term fuel remains steady around -1%. Being that it's only 1 bank, I doubt there's an issue with the MAF.

    I've got two NTK 02's on order and am hoping that I'll get some feedback on my parking lot scanner diagnosis. Of course, I could have wiring concerns but I'll begin by replacing the O2. I'm fairly competent with a DVOM and a labscope so that shouldn't be too horribly awful to diagnose if necessary.

    I'm sick and tired of chasing misfires on late model Ford engines. It's absurd that there's no misfire codes on some of these. We have two 5.4's in the shop right now for longblock replacement, just had a timing set, phasers and VVT solenoids on another 5.4 truck, rough idle on a 3.5 Ecoboost '15 1/2 ton and an intermittent flashing CEL miss on a 4.0 Explorer.

    Comments, concerns let me know.

    Thanks guys,
    Mike
     
  2. 99GTMichael

    99GTMichael New Member Established Member

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  3. 99GTMichael

    99GTMichael New Member Established Member

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  4. 99GTMichael

    99GTMichael New Member Established Member

    Messages:
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  5. 99GTMichael

    99GTMichael New Member Established Member

    Messages:
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    Location:
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  6. 99GTMichael

    99GTMichael New Member Established Member

    Messages:
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    Joined:
    Nov 24, 2015
    Location:
    Missouri
    I replaced the 02 sensors today and that fixed the issues with B2S1 not switching properly. Thank goodness I have access to a lift because that would have been miserable without it!

    I still have what I consider a rough idle, but I cannot feel any misfire running out down the road at this time. I am suspicious of my Motorcraft coils that I put on two years ago. I am considering trying the MSD coils as my experience with MSD products for a small block Ford have been positive.

    Anyone have any input on the MSD coils?

    Thanks guys
    Mike
     

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