Misfire code p0303 while under boost

Discussion in 'Mazdaspeed 3 Troubleshooting' started by virtualsurfer, Nov 18, 2020.

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  1. virtualsurfer

    virtualsurfer Greenie N00B Member

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    Hello all!

    I've had my speed3 for some time now but I'm new to this forum specifically because I could use some outside advice.

    My car has developed a cylinder 3 misfire (p0303) while under boost around 3.5k-4k. The car runs perfectly fine under normal driving circumstances and the idle is solid. I've replaced my plugs with NGK 6509 gapped to 0.032" as well as swapped coil packs and the misfire remained in cylinder 3. The old plugs looked good across the board save for some minor gap changes. The fuel pressure appears to be in order and the fuel pressure relief valve appears to be functioning properly too.

    I've noticed when I'm going into boost a cloud of black smoke comes out of the exhaust; could this be related? I know these cars tend to run rich under boost but this was an abnormally large cloud.

    Should I begin looking at the injectors and valves at this point?

    Thank you in advance.

    EDIT: stock except for COBB SRI and HPFP internals. VVT done 3k miles ago.
     
    Last edited: Nov 18, 2020
  2. The_big_dill

    The_big_dill Greenie Member

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    Do a compression test wet and dry, and report the numbers.
     
  3. virtualsurfer

    virtualsurfer Greenie N00B Member

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    I'll do a compression test when I eventually get some time off work. Anything else I should check while I'm doing that?
     
  4. The_big_dill

    The_big_dill Greenie Member

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    Take note of the tip of every plug you pull, does cylinder 3 plug look different than the others? If possible get a picture.

    If you moved your coils and plugs around and the missfire remained in the same cylinder, then the most probable candidates are an injector, injector seals or compression.

    Since a compression check is far easier to do and will be the bigger factor on decision making for the future of your car, it is advised you start with that.
     
  5. virtualsurfer

    virtualsurfer Greenie N00B Member

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    I only got a picture of plug #3 but they all looked similar. Apologies for the out of focus picture.
     

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    virtualsurfer, via an iPhone, Nov 20, 2020
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  6. virtualsurfer

    virtualsurfer Greenie N00B Member

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    Alright compression test results came back 190 across the board. It’s worth noting I used a $30 advance auto compression test kit but the numbers are all consistent so I assume it’s okay?

    I’m going to pull the intake manifold soon to check out the injectors/seals and give the valves a good cleaning (at 160k i’m sure they’re caked).
     
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