Loss of MPG after installing aftermarket Catback
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Loss of MPG after installing aftermarket Catback

This is a discussion on Loss of MPG after installing aftermarket Catback within the Focus RS Discussions forums, part of the Focus RS Forums category; Recently installed an MBRP catback. Theres a small leak at the midpipe slip joint. I have a Mountune unit on the way to replace it, ...

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    Supporting Member Hankbobo's Avatar
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    Loss of MPG after installing aftermarket Catback

    Recently installed an MBRP catback. Theres a small leak at the midpipe slip joint. I have a Mountune unit on the way to replace it, but Ive noticed something: my mileage has gone to **** since install, and I dont know if its just the leak since its after the O2 sensors.

    I thought it was my imagination but I cleaned the MBRP tips up to a perfect shine and within 48 hours they were absolutely covered in carbon. Like, black black. Im on the stock tune, and I know it runs rich compared to AM tunes, but I've never seen soot collect this quickly.

    Has anyone else noticed a decrease in mileage after installing an aftermarket catback on a stock tune? Fearing I have a leak somewhere at the DP/turbo flange...

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    Another possibility is that it didn't affect you at all. We are at the point in the season where gas companies are switching over the winter formulas, and I too have noticed a drop in mileage, without any mods. Happens every year.
    Hankbobo and lx_2nv like this.

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    You getting high LTFT, like 15% or greater when not under boost?

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    Aftermarket exhaust systems also cause a bit of loudpedalitis...
    XRJoe, cntrdctn86, loubif and 2 others like this.
    Take everything I post with an MDOT truckload of salt.
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    Quote Originally Posted by Regulated View Post
    You getting high LTFT, like 15% or greater when not under boost?
    No way to tell. AP not installed. One of the sad realities of having to stay stock

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    Quote Originally Posted by onestep View Post
    Aftermarket exhaust systems also cause a bit of loudpedalitis...
    Nah. I have a lead foot all the time, way before I put the exhaust on. Wouldn’t explain the excessively fast carbon buildup. Never happened before that

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    What were your average MPGs before the install and what are they now?

    Can you log AFR during cruise and WOT? What were the numbers before? A leak post cat flange is unlikely/near impossible to affect AFR readings and fueling demands.

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    Quote Originally Posted by XRJoe View Post
    What were your average MPGs before the install and what are they now?

    Can you log AFR during cruise and WOT? What were the numbers before? A leak post cat flange is unlikely/near impossible to affect AFR readings and fueling demands.
    Can’t log AFRs, no AP installed. Was getting about 17.5 consistently before, now sitting at about 16.3ish. Checking the turbo/DP flange this weekend.

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    Winter fuel does reduce fuel economy.

    Look at my chart for the past year. You can see the general trend from October through April is slightly worse than the warmer months.

    http://www.fuelly.com/car/ford/focus...5650/fuelchart
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    You must drive like a mad man. I average 24.5.

    Quote Originally Posted by Hankbobo View Post
    Can’t log AFRs, no AP installed. Was getting about 17.5 consistently before, now sitting at about 16.3ish. Checking the turbo/DP flange this weekend.

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