P0303 CEL - Best Shop To Diagnose?

Discussion in 'Modifications And Maintenance' started by Erik, May 29, 2019.

  1. Erik
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    Erik Well-Known Member

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    Cylinder #3 is misfiring pretty bad, and it started just a couple of days ago. It's a 2012 STi with 90,000 miles. No aggressive mods have been done, only a Cobb AP running Stage 1 91 octane.

    I have cleaned the MAF sensor, run injector cleaner through my fuel and reset the ECU. I think I probably need to bring it somewhere for further diagnosis. If it were your car, where would you take it?

    I have been OCD about maintenance on the car, but have never changed the fuel filter or the coil packs. I did the spark plugs at 67,000 (Denso IKH20 stock temperature, .040 gap).

    Thanks in advance!
     
  2. pbedroske
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    pbedroske Well-Known Member

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    I don't know much about 2012's but .040 gap seems pretty big to me. Also, are denso plugs stock or NGK? I would go NGK assuming they're stock and gap at .027 to .030. Check coil packs for separation of the laminations. Though perhaps you have a bad batch of fuel. And a fuel filter is cheap too.
     
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  3. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    JMAuto.

    Self diagnosis:
    With car idling, pull oil cap and look for smoke.
    If no smoke, pull plugs and inspect (looking from the front of the car, left/passenger side closest to you, next to the airbox is cyl 1 and cyl 3 is behind that). If plugs and gaps look good (.040 is pretty big as mentioned) swap plugs and see if misfire follows the plugs. Can do the same with coil packs if needed.
    Rent a compression tester if you don't have one already and check compression on cyl 1 and 3 while you have the plugs out. Piston ring damage will show up as something like 140psi on cyl 1 and 0psi to very low on cyl 3. Higher psi in cyl 3 (but out of spec compared to 1) probably indicates a valve leak.
    If everything still checks out you can repeat the same as you did for the plugs but with the injectors.
    At that point, if you're still clear, it's probably wiring/electrical.
     
  4. Erik
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    Erik Well-Known Member

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    It's these: https://www.fastwrx.com/products/denso-iridium-plugs

    And I guess I might be wrong on the gap. I noted it in my maintenance history, but I don't think that is the stock gap on these plugs so I don't know where I got that number.

    I am not quite sure how to check the coil packs for what you said, and where can I buy a fuel filter? fastwrx doesn't carry then and neither does rallysportdirect. Is it as simple as buying one from Napa or any auto parts store? I tend to like to buy from companies that focus on the these types of cars versus just generic parts places.
     
  5. Erik
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    Erik Well-Known Member

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    Thanks! No smoke when the car is idling - at least idling when it is already hot. Should I do this with the car cold as well?

    I could try the other things that you mentioned, but was also hoping you might know of a good local trustworthy shop that I might be able to bring it to. I am comfortable working on my car (so far I have done everything myself) but time is the problem now...I don't have a day that I can dedicate to doing the troubleshooting myself.

    So if I didn't want to bring it to the dealer - where would you go?
    And if I did end up bring it to a dealer - which one? I bought from Scott Putnam at Morries Minnetonka. The Bloomington dealer is super close to where I work, and I have some connections with the Burnsville dealer that might be beneficial.
     
  6. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    Re: dealerships, I personally have not had any positive experience with any service departments so I can't give you a recommendation there.

    Good luck and I hope it's something simple.
     
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  7. Erik
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    Erik Well-Known Member

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    Thanks - anybody have experience with MAPerformance? I don't know if they do repairs or not, but they are much closer than JM Auto. Thoughts?
     
  8. 06LGT
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    06LGT New Member

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    To my knowledge, MAPerformance no longer works on customer vehicles due to their growth in business as an online parts distributor. They also tend to be more focused on Mitsubishi nowadays.

    +1 for JM Auto. Jessie and his team do amazing work. I would suggest calling tomorrow (Monday), however Proving Grounds is this coming weekend so I'm guessing they will be booked out by at least 2 weeks.

    In the meantime, Sorenson Auto is Savage also does Subaru repair. Perhaps they would be able to run some diagnostics much sooner. https://www.sorensonautomotive.com
    https://www.facebook.com/Sorenson123/
     
    Last edited: Jun 2, 2019
  9. 06LGT
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    06LGT New Member

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    I, too, would not take the vehicle to any dealership service department (especially Morries).
     
  10. pbedroske
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    pbedroske Well-Known Member

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    [​IMG]
    These for example have corrosion/separation of the laminations. These are similar to the coil on spark coil packs we have on Subies. Any rust or corrosion could cause a short internally or to the block. When I had an Acadia, one coil shorted internally (new within a year) and blew out the fuse for the whole bank causing severe misfire. There were no physical signs of damage to the coil pack. I had to get it towed and diagnosed, also found the misfires fouled the catalytic converter$.
    If image is not loading, google coil pack corrosion.
     

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