Cylinder 3 misfire and low/erratic fuel pressure

Discussion in 'Mazdaspeed 6 Troubleshooting' started by AdrianMS6, Aug 26, 2022.

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  1. AdrianMS6

    AdrianMS6 Greenie N00B Member

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    My MS6 has developed a misfire on cylinder three and threw both p0300 and p0303 codes.

    The spark plugs are all good and gapped within factory spec, I tried checking the coil packs. But after clearing the code to see if the cylinder switched along with the coil pack, the check engine light has not come back on though it is clear the car is running poorly and still misfiring.

    I borrowed a nicer scanner to run some diagnostics and found the car has really low fuel pressure. It's bouncing between 60 and 200 psi on idle and only got up to 1400 at WOT. I spliced in a fuel pressure gauge to check the in-tank fuel pump as I have just changed that out for a Deastchwerks one and I was getting ~60 psi before the HPFP.

    I've read a little about the fuel pressure relief valve causing issues and a cylinder 3 misfire, does that seem like the culprit here? Or is it probably the HPFP itself?

    Any advice is appreciated.
     
  2. SyntheticAtmosphere

    SyntheticAtmosphere You only get ONE ride, ONE ticket, ONE time! Silver Member

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    Your pressures post ITFP are low. Do not WOT the car till you have repaired the issue.
    Here is a good read about the HPFP and how it works...
    CS HPFP Troubleshooting
    It is possible that your previous issue was related to this issue and it would be valuable to test the whole system and consider replacing and upgrading. Seeing as this could be a spill valve, relief valve issue or mechanical issue you should run some tests and look into pulling the pump apart. The spill valve can be cleaned and inspected. Now would be the time to check for damaged internals or upgrade if stock.
    Does your car have upgraded internals?
    With the age of these cars, Im finding tasks are switching from repairs to rebuilds.
     
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  3. AdrianMS6

    AdrianMS6 Greenie N00B Member

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    Yeah, I'm not gonna be driving the car unt I get this all sorted out. I don't want to break it more.

    I just replaced the ITFP and fuel pump relay, which seemed to fix my previous issue. I haven't had any issues with the ITFP since.

    The car is practically stock, so no upgraded internals.

    That's definitely a good read and very helpful, so thanks for sharing that. I'll see what else I can test, and I will definitely be pulling the HPFP apart soon.

    Do you have any recommendations for upgraded fuel pump internals? I am in CA so sadly have to stick to CARB legal stuff.
     
    AdrianMS6, via a mobile device, Aug 27, 2022
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  4. SyntheticAtmosphere

    SyntheticAtmosphere You only get ONE ride, ONE ticket, ONE time! Silver Member

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    Internals arent affected by CARB. However to fully benefit from them you need to adjust the max pressure via tuning.
    I have Autotechs, because they were the gold standard at the time, but any reputable brand is good. Corksport also has them.
     
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  5. AdrianMS6

    AdrianMS6 Greenie N00B Member

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    That's good to know. I'm coming from a pre-1975 project car where emissions didn't matter at all, so I am still trying to wrap my head around what I need to worry about and what I don't when it comes to CARB.

    Thanks for all the advice, I appreciate it.
     
  6. AdrianMS6

    AdrianMS6 Greenie N00B Member

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    Posting one last update to hopefully help someone in the future.

    I changed out the fuel pressure relief valve and it solved the issue. Fuel pressure is up to ~450 psi at idle, no more cylinder 3 misfire, and car gets to 1600+ psi on just a tap of the throttle.
     
    AdrianMS6, via a mobile device, Sep 2, 2022
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  7. Haedenjames

    Haedenjames Greenie Member

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    I have the same exact issue. I have the fuel rail off and was gonna swap injectors around to test as a last result but just figured out I can blow through both ends of my relief valve. That means it’s toast right?
     
    Haedenjames, via an iPhone, Oct 29, 2022
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  8. Awafrican

    Awafrican Moderator Gold Member

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    Unless you somehow blow at >1800 psi then yes it's toast
     
    Awafrican, via a mobile device, Oct 29, 2022
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  9. Haedenjames

    Haedenjames Greenie Member

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    So how the hell does it cause a misfire on only cylinder 3 I’m so confused. Shouldn’t it affect all cylinders?
     
    Haedenjames, via an iPhone, Oct 29, 2022
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  10. Awafrican

    Awafrican Moderator Gold Member

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    Flow to cylinders isn't balanced also failed prv might not be your only issue
     
    Awafrican, via a mobile device, Oct 29, 2022
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