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Thread: P0300

  1. #1
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    P0300

    Good day all. i have a STOCK 2000 Chevy Silverado 5.3 that has a p0300 and LT fuel trims are 25% both banks and misfires on cylinders 7 and 8 occasionally 1. When i remove the MAF sensor i have zero misfires and it appears to ideal a little better. LT fuel trims drop a little. Any suggestions?
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  2. #2
    Advanced Tuner IARLLC's Avatar
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    Intake manifold gasket is the most likely cause.

  3. #3
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    why do you say that?

  4. #4
    Senior Tuner cobaltssoverbooster's Avatar
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    un metered air causes o2 to be drastically lean which makes your trims go high positive. typically the intake gasket goes out and leaks air into both sides which is why both are reading that way.

    stock tune the maf isnt active until 4000 rpm so all unplugging it does is removes the iat circuit thats usually in it and forces open loop due to thrown codes which means the trims aren't active. in 200 you might have the canister maf which might not have an internal iat, but either way disconnecting it forces open loop due to triggered codes.
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  5. #5
    Tuning Addict 5FDP's Avatar
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    Smoke test for leaks.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

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    would the lean condition cause cylinders 7 and 8 to misfire. and also why when removing the maf would the misfires stop? not questioning you all just trying to learn as i go

  7. #7
    Senior Tuner cobaltssoverbooster's Avatar
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    Unplugging it triggers open loop and the base fuel table is commanding fuel instead of the trim system.

  8. #8
    Senior Tuner Russ K's Avatar
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    Quote Originally Posted by GMT800RCSB View Post
    would the lean condition cause cylinders 7 and 8 to misfire. and also why when removing the maf would the misfires stop? not questioning you all just trying to learn as i go
    Yes, a lean condition due to intake gasket leak (very common on the 5.3 & 6L trucks) can cause 7 & 8 to misfire. Any Maf DTC disables the misfire counters.

    Russ Kemp

  9. #9
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    Quote Originally Posted by cobaltssoverbooster View Post
    un metered air causes o2 to be drastically lean which makes your trims go high positive. typically the intake gasket goes out and leaks air into both sides which is why both are reading that way.

    stock tune the maf isnt active until 4000 rpm so all unplugging it does is removes the iat circuit thats usually in it and forces open loop due to thrown codes which means the trims aren't active. in 200 you might have the canister maf which might not have an internal iat, but either way disconnecting it forces open loop due to triggered codes.
    There is so much wrong in this post.

  10. #10
    Senior Tuner cobaltssoverbooster's Avatar
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    Quote Originally Posted by 2xLS1 View Post
    There is so much wrong in this post.
    i dont see how. that happened to be exactly what i experience last year after i ruined one intake gasket on my yukon during an valve train refresh.

    i dont mind being called out as wrong, but where is the return comment on why?
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    2016 GMC 3500 Denali Diesel
    2000 Ford Mustang - Top Sportsman

    Ecotec help can be found here:
    LNF-http://www.hptuners.com/forum/showth...357-LNF-Guides
    LSJ-http://www.hptuners.com/forum/showth...965-LSJ-Guides

  11. #11
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    Quote Originally Posted by cobaltssoverbooster View Post
    i dont see how. that happened to be exactly what i experience last year after i ruined one intake gasket on my yukon during an valve train refresh.

    i dont mind being called out as wrong, but where is the return comment on why?
    Quote Originally Posted by cobaltssoverbooster View Post
    un metered air causes o2 to be drastically lean which makes your trims go high positive. typically the intake gasket goes out and leaks air into both sides which is why both are reading that way.

    stock tune the maf isnt active until 4000 rpm so all unplugging it does is removes the iat circuit thats usually in it and forces open loop due to thrown codes which means the trims aren't active. in 200 you might have the canister maf which might not have an internal iat, but either way disconnecting it forces open loop due to triggered codes.
    What is bolded is wrong.

    I've seen you say a lot that the MAF isn't active below 4000 rpm when in fact it is quite active in a blended mode along with the VE table. And unplugging the MAF does not force open loop but it causes the vehicle to run in speed density.

  12. #12
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    just recently found a rotted egr pipe on my 02 2500 6.0.
    didnt really show with carb/rake clean spray.
    looked horrendous with smoke test though.
    before i fixed it and started tuning, LTFT were 25%.

  13. #13
    Senior Tuner cobaltssoverbooster's Avatar
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    Quote Originally Posted by 2xLS1 View Post
    What is bolded is wrong.

    I've seen you say a lot that the MAF isn't active below 4000 rpm when in fact it is quite active in a blended mode along with the VE table. And unplugging the MAF does not force open loop but it causes the vehicle to run in speed density.
    Thanks for replying. I was taught years ago to view high rpm disable as a threshold where maf wasn't or was used.
    It is possible my older ways aren't relevant these days.

  14. #14
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    Thank you all for the responses. I just bought a set of FELPRO gaskets today and will get it installed and repost any new log data. Im very interested in learning the stock system of whats normal. I believe (tell me if im wrong) that if i understand fuel trims and o2 sensor maf sensor data that would be the base info for tuning.