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  1. J

    Mystery P0300 root cause?

    *** solved*** The drive belt tensioner I replaced 4x months ago already failed and was causing belt slap or slip catch seneraio to trigger crankshaft sensor to report false misfires! While I was waiting on salvage test pcm I thought I'd address the accessory noise that started recently and...
  2. J

    Time to replace the PCM?

    Don't use solider anything electrical that is subject to vibration as thats a stress point that can break because of stiffness, use crimp connectors and heat shrink over it.
  3. J

    Mystery P0300 root cause?

    Replaced ASD relay early on in this adventure. Today I tested control from pcm to asd relay, that's good Checked that in fact there is power going from ASD to injectors and coils at startup abd. My issue is the pcm isn't ground switching the injectors and coils as expected
  4. J

    No deceleration mode in overdrive?

    Bump
  5. J

    Mystery P0300 root cause?

    Bump...
  6. J

    Mystery P0300 root cause?

    You mean keep replying to this post or make a new one?
  7. J

    Mystery P0300 root cause?

    In the month I've been trying to fix the P0300 code, I also developed a long hot start issue. If car is warm and I turn off for 20 minutes and restart it takes 3 seconds vs the usual <1 second to start. I'm not sure if it's related but since it's easier to reproduce problem in my driveway I...
  8. J

    Mystery P0300 root cause?

    In the month I've been trying to fix the P0300 code, I also developed a long hot start issue. If car is warm and I turn off for 20 minutes and restart it takes 3 seconds vs the usual <1 second to start. I'm not sure if it's related but since it's easier to reproduce problem in my driveway I...
  9. J

    Mystery P0300 root cause?

    Yeah I'll just replace it as just about everything needs replacing on this car, I'm restoring the jeep apparently vs making car payments, a pain in the neck but still cheaper then new car and good for me to step my wrenching game up as I don't ever plan on buying new again with today's car...
  10. J

    Mystery P0300 root cause?

    So the engine would receive air not fuel air vapor and possibly misfire? Why wouldn't it misfire all the time then? Why only going downhill coasting? .why wouldn't I have error code for evap? What about temporary disconnect purge valve from intake so it's venting outside vs into intake, see if...
  11. J

    Mystery P0300 root cause?

    Hi, mine shows fine in scan tool live data and freeze frame and in recorded data
  12. J

    Mystery P0300 root cause?

    Hi Pcv was replaced It's got electronic throttle body plate and feels tight no play in it, was cleaned also You mean evap canister? No evap codes, how would I test canister? How would that cause p0300? I checked the cam crank correlation waveform with known good, it's right on, without any...
  13. J

    Mystery P0300 root cause?

    I know the module sometimes will set a dtc code, but sometimes they fail without a code, like a loose component inside or cold joint and then the car does different things randomly depending on temperature or vibration. Or capacitor or driver, etc is burnt out
  14. J

    Mystery P0300 root cause?

    I'm going to hack the pcm and know it's hardware and firmware as soon as I get a chance to setup a breakout box and J box and whatever software I need as it's ridiculous that I or any mechanic has no idea what is going on inside a critical component. I have no understanding of its inner workings...
  15. J

    Mystery P0300 root cause?

    It's stupid the factory service manual doesn't explain the pcm inner workings or give a logic tree so a person can know exactly what inputs and outputs and decisions the pcm is suppose to make. The factory service manual just says under hard deceleration the pcm switches to deceleration mode...
  16. J

    Mystery P0300 root cause?

    So you're saying w Cruise off and overdrive on you can feel it compression braking vs feeling like you're in neutral coasting?
  17. J

    Mystery P0300 root cause?

    Hi, So your scan tool doesn't show pcm entering open loop drive deceleration mode normally driving downhill unless Cruise control is active? You can feel it to even without scan tool, it feels like engine braking vs no engine braking. Feels like if it were stick shift and went downhill without...
  18. J

    Mystery P0300 root cause?

    Hi, as I wrote the intake and exhaust was smoke tested for leaks, and long term fuel trims around 6% or so no indication of vaccum leak
  19. J

    Mystery P0300 root cause?

    Thanks haha, I made a big check list as just about anything can cause misfires. I actually still have more tests to do, like borescope cylinders abd injectors ports. And Leakdown test again of all cylinders, just for the heck of it I'm going to swap everything from cylinders 1, to 6 and see if...
  20. J

    Mystery P0300 root cause?

    2007 jeep liberty v6 3.7L automatic 190k miles Hi I'm chasing down a difficult P0300 code with CEL flashing but it only happens when coasting downhill over 50+ mph. I've checked and tested and replaced dozens of things in the process. I also am now getting random long recently might be related...

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