Lower MPG after spark plug change
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This is a discussion on Lower MPG after spark plug change within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Brief history of my WRX. I bought it back in FEB 2011, it appeared to be well maintained when I ...

  1. #1
    Registered User pete25r's Avatar
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    Lower MPG after spark plug change

    Brief history of my WRX. I bought it back in FEB 2011, it appeared to be well maintained when I took over. It would backfire when the clutch was in to change gears and there was a constant very faint smell of gas in the exhaust. It had about 75k on the odometer. So with all that I thought a good thing to do would be to replace the plugs.

    Mods on car: 3" turbo back exhaust, cobb "cold" air intake, big k&n filter, probably some type of Accessport map loaded - cobb usb adapter thing was on OBD-II port when I bought the car.

    Ok so changed out some NKG PFR6Gs (double PLT) with some NKG BKR6EIXs (iridium).
    The old ones came out, looked dirty/oil around the threads and discolored around the ground electrode. Gap on old plugs was between .032 and .035

    I checked the gap on the new plugs and it averaged .026. I read on here and other sites the gap was supposed to be between .028 and .031. I gapped the iridiums to .029. And yes I did so very carefully as to not damage the gap surfaces.

    My problem seem to be my MPGs took a big hit. I was getting just shy of 300 miles to the tank and now its around 250 miles. I just though of something, might have answered my own question/problem. I have been driving it more spirited-like and it has been hot as youKNOWwhat around here so the A/C has been on a lot.

    My questions are: does my spark plug change sound like it would be dropping my MPG?
    is it possible to tweak a cobb map to were you would have to gap your plugs differently?
    when is this stupid southern heat wave gunna end?

    thanks for your time...

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    Registered User jd92677's Avatar
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    Do you personally know the person that owned the car before you? It would probably be good to know if the car is tuned. Sound like you've been running rich if you're backfiring. If you have a catless TBE, the smell may be normal. Can plugs affect mileage? Yes but not that much. What worries me is that you have a COBB intake with a K&N filter, and some kind of tune (maybe). I would be sure of what you have first, then go from there.
    Stink eye mob #727
    2011 WRX DGM Hatch, Protuned Stage 1

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    Registered User pete25r's Avatar
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    know?

    Don't know the previous owner and dealership wouldn't give up any contact info.

    Why a K&N on a Cobb intake? Well K&N is less expensive and had a larger surface area.

    Just remember something, I left off a mod. There is a turboXS blow off valve, the "pssst" type - dumps to atmosphere. I heard that might make you run a little rich, it that true?

    How can I tell what tune is on the ECU without an Accessport?
    Would an open source cable work? Like a Tactrix or something?

  5. #4
    Registered User jd92677's Avatar
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    Yes a tactrix cable would work. A BOV will cause the car to run rich, especially between shifts so that could explain that. Personally, I would ditch the BOV, it's just causing problems, get a tactrix, see what your tune looks like, and make sure it's compatible for the mod you're running.
    Stink eye mob #727
    2011 WRX DGM Hatch, Protuned Stage 1

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