Misfire under WOT

Discussion in 'Mustang Chit Chat' started by Vallee, Jun 12, 2018.

  1. Vallee

    Vallee forum member


    I am at a total loss with resolving this issue.

    Without getting into too much boring details, here’s a general background of my situation. After I finished building my engine late last summer I contacted JDM and asked them if they could supply me a safe “turnkey” tune until I could get to a dyno and they did. The car ran ok however the tune was very rich and kept throwing codes (P0175) and subsequently I could not get it to pass NYS emissions. So it was suggested that I try reaching out to Lito, so I did. He is great, we got the car to pass emissions and run decent and now we’ve been working on slowly dialing the car in. Along the way I’ve had this annoying hesitation (misfire) issues with WOT acceleration under a load. The best way I can explain it is, it acts like you cut the ignition power and then turned it back on and off and on again…

    Over the past winter I did the following things to try to fix the issue:

    - Verified all vacuum lines for leaks, and checked all wire connections.
    - Changed out the MDS coil packs back to FORD OEM’s P/N: M-12029-3V (Brown Boots)
    - Replaced the old 6G with a new DOB Denso high output 165A alternator.

    Fast forward to this spring, Lito asked me to get some logs in a 3rd gear WOT run, boost was reporting a tad over 15psi and the car started breaking up again (probably started at around 3400 rpm and up), just as it did last fall… Just as before the wrench light flickered and the car went into limp mode. No CEL codes were stored.

    Lito and I reviewed the log and saw nothing out of the ordinary. Very Puzzling…

    - @ WOT the AFR between 10~11:1
    - Batt Volts 13.6v
    - Spark Adv 12

    Per the request of Lito I pulled the plugs for a visual inspection, overall they looked ok but did have a hint of white residue around the electrode, and all were visually consistent with each other. Gaps are verified at .028. Plugs are FRPP NGK Iridium p/n ZNAR7AIX 7554

    Image of the plugs below:
    20180603_094532.jpg 20180603_094610.jpg

    Screen shot image of the WOT Log with misfire is attached below:

    I recently checked all chassis ground connections. As I had the engine bay smoothed and painted while the engine was out, and I was thinking the ground lugs may not be getting proper ground contact to the chassis and causing an electrical issue. However, I retested and although it did seem to help the misfire is still there.

    I replaced the battery (found my old red top was not holding charge over time), have not yet tested it.

    At this point I am thinking it could be a crank sensor? Clogged cats? Bad driver in the PCM?? Clogged injectors? Fuel pump failing (not sure it’s a fuel issue)? Perhaps the plugs are just junk? Also, surprisingly, it appears that the MSD COP's seemed to respond better then the OEM’s…

    For what it’s worth, my old engine would misfire kind of like this whenever I down shifted and stabbed the throttle. Only now it’s all the time under WOT.I just thought it was the tune and lived with it... Now I am not sure..

    For Reference - Here is a basic list of my build:

    - 2005 Mustang GT (Standard 5sp)
    - M-6009-A463SC FRPP, 4.6L 3V Aluminum Forged Short Block 8.7:1 CR for SC
    - 127350 Comp Cams, Stage 2 Blower Camshafts
    - Vallee’s Custom 3” 8 rib blower pulley, I see just under 16 psi of boost.
    - 918018 ATI, Super Damper Serpentine Series 8rib Harmonic Balancer.
    - M-6050/6049-N3VPA, Ford Racing stage-1 CNC ported Modular Cylinder Heads
    - M-12405-3V12MM FRPP NGK Iridium #ZNAR7AIX-7554 Spark Plug - 3V 12MM gapped at .028
    - VMP’s TVS high flow elbow.
    - Roush Eaton TVS/R2300 Supercharger
    - GT500 TB & Fuel Pump
    - Kooks 1-3/4 LT Headers with the Green Catted 3” H-Pipe.
    - ID1000 injectors with OEM GT500 fuel rails
    - Tiresize is 255/45zR18 103W
    - Rear DifGear ratio is 3.73:1

    Here is the link to my build thread:

    I am open to any ides and opinions... thanks Guys!

  2. Slick4.6

    Slick4.6 forum member

    Take a look at the maf? I had a weird issue similar to that on an older car of mine, idle fine and ran fine, wot it would break up and ended up being the maf, no cel either. Just a thought.
    46addict likes this.
  3. NickD87

    NickD87 forum member

    I've had a similar feeling before and it was the crappy tracktion control system cutting spark
    Next I'd likely change plugs

    Sent from my Nexus 9 using Tapatalk
  4. 07gts197

    07gts197 forum member

    What is analog and what units is it in? Im guessing afr but Im not aware you can datalog afr with livelink.

    More pids need to be monitored including knock sensor activity, fuel pump duty cycle, fuel rail pressure, maf voltage and both short and long term fuel trims.

    For what its worth it sounds like your fuel pump cant keep up but you should log more to verify that.
  5. Gabe

    Gabe Whippled Coyote

    Most of the time the wrench light will come on it will be for the throttle body.
  6. 46addict

    46addict 13726548

    I think Slick4.6 is barking up the right tree in saying that it could be a the MAF. I'm not saying you have a fueling issue but it would help to see what fuel pressure and pump duty cycle are doing when dealing with problems like this. The PIDs for that are "pres drop f inj" and "fp duty cycle" which I noticed were unchecked.

    If the throttle position sensor voltage variations do not match the MAF sensor voltage curve, you will most likely get the wrench light.
  7. NickD87

    NickD87 forum member

    He's tuning with Lito,
    I know Lito logs fuel pump throttle position etc so doubting anything like that

    Sent from my Pixel 2 XL using Tapatalk
  8. 46addict

    46addict 13726548

    I said in my post I'm not questioning the fuel system. It was just out of curiosity.
  9. Marble

    Marble forum member

    I had somthing similar happen and it ended up being bad fuel. I would describe it just like you said, being that it's been going on for fire a really long time, fuel seems like an unlikely candidate.

    My vote is for something in the MAF, TB or a poor ground/damaged wire.
  10. RocketcarX

    RocketcarX 95% of my weight is fuel

    Maybe start with new plugs opened up to .035
    Also a video of the car breaking up would help a lot
    A crank sensor isn't a bad idea, but if it was cutting out you would see it in the logs I would think
    An OEM MAF sensor take off can be purchased on eBay for like $20 shipped for testing purposes
    What does Lito think is going on at this point?
  11. Vallee

    Vallee forum member

    Thank you everyone for the replies. Sorry I haven't had time to respond to them. Very busy the last few days. When this happened last year I thought it was bad gas but now I'm running 100 octane to see if it helps and it doesn't change a thing.

    Lito thinks poor groud conection, plugs or possibly somthing electrical.

    Here is whats on order for this weekend:

    Re-inspect all electrical connections
    New Battery
    New crank position sensor
    New MAF
    New GT 500 TB
    New plugs FRPP/NGK ZNAR7AIX 7554
  12. Gabe

    Gabe Whippled Coyote

    I wouldn't do all those things at once, you'll never know what fixed it, if it's fixed after shotgunning all those parts into the car.

    I'd start with easiest to replace and/or cheapest and work up the list.
  13. Vallee

    Vallee forum member

    I agree with you however time is of the essence and right now I only get about 4 hours a week to mess around with the car, it's easier to Jack the car up and do all these changes at once.

    Yes, It would be nice to know which one of those items if any fixes the issue however at this point I'm just looking to get the car running.
    Last edited: Jun 14, 2018
  14. 07 Boss

    07 Boss forum member

    I might try and widen the plug gap just a hair. Granted I don't run that much boost but the last time I had them gapped that small I ran into some issues with stumbling. I run them at .032.

    Another wild idea that we used to use back in the day with hard to ignite fuel mixtures was to side gap the plugs. If you're going to swap plugs anyways I might try to side gap the old ones first and see if that makes a difference. Granted we used to do this with our nitrous cars, but it will unshroud the spark.
  15. RocketcarX

    RocketcarX 95% of my weight is fuel

    Open the plug gap and add a ground from the engine to the battery directly
    Marble likes this.
  16. 07 Boss

    07 Boss forum member

    oops, please delete
  17. Pentalab

    Pentalab forum member

    With his 16 psi of boost, his .028" is about right. .035 for 5-6 psi. .032 for 8-9 psi.

    I suspect that new plugs may well solve his issue.
  18. RocketcarX

    RocketcarX 95% of my weight is fuel

    all of those gap settings sound too tight for the given boost ratios. You should not have to touch the plugs at 5-6 PSI for sure, and at 8-9 PSI I would think its still a maybe.
  19. Vallee

    Vallee forum member

    Well guys I spoke too soon. I changed the throttle body, MAF, verify that the harmonic balancer was seated and torqued properly, new
    spark plugs gaped at .029" verified all gound connections and took it out for a test spin, it seem to run quite well. Today I went for another much longer drive, a steady Cruise 2500 rpm @ 55 miles per hour (no boost) and started skipping AGAIN! Heat seems to make it worse, its like 90 today, I captured it all on my GoPro and I'll try to post a video later tonight. I'm beginning to think it's a fuel problem? I'm going to pull the plug tonight to inspect them. Cylinder #1 logged a misfire, there are also random misfire codes. It acts like it's a fuel problem??? like you took your foot off the throttle... Shut the car off wait a few seconds, start it up and take off and it ran fine I have no clue WTF is going on. Time to ditch the 4.6... 20180617_124545.jpg
  20. travelers

    travelers Senior Member

    How did you verify the grounds are good?