Pre and Post Mods afterfires
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This is a discussion on Pre and Post Mods afterfires within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; I've read some about afterfires which are usually caused by unburned fuel in the exhaust, BUT... all I've read talk ...

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    Registered User Julio Bro!'s Avatar
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    Pre and Post Mods afterfires

    I've read some about afterfires which are usually caused by unburned fuel in the exhaust, BUT... all I've read talk about modded vehicles. I have experienced this since new.

    Although this is like, a couple of times a year, it has always been during WOT acceleration and 1-2 shifting, and sometimes a 2-3 shift. Mine is a 2009 WRX and the first "mod" came after a year with an OEM style K&N filter. Then came a Dynomax VT muffler in the third year, now I've been a year with a KS Tech cold intake. The highest octane around is 91, it's a lucky find to get 92, and there's only one station that offers 93, very expensive here.

    My usual fuel is 91 octane and I must emphasize that the first afterfire happened during the first year, completely OEM.

    So, I undestand afterfires are not good, but seems to be a usual thing with these engines, correct? But, why when OEM?

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    Registered User subypete's Avatar
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    Quote Originally Posted by Julio Bro! View Post
    I've read some about afterfires which are usually caused by unburned fuel in the exhaust, BUT... all I've read talk about modded vehicles. I have experienced this since new.

    Although this is like, a couple of times a year, it has always been during WOT acceleration and 1-2 shifting, and sometimes a 2-3 shift. Mine is a 2009 WRX and the first "mod" came after a year with an OEM style K&N filter. Then came a Dynomax VT muffler in the third year, now I've been a year with a KS Tech cold intake. The highest octane around is 91, it's a lucky find to get 92, and there's only one station that offers 93, very expensive here.

    My usual fuel is 91 octane and I must emphasize that the first afterfire happened during the first year, completely OEM.

    So, I undestand afterfires are not good, but seems to be a usual thing with these engines, correct? But, why when OEM?
    I would not call afterfires "usual" at all. Especially with a completely stock exhaust system, with all its cats. The fact that it is ocuring between shifts leads me to believe something might be going on around your bypass valve (BPV). However, for you to be running rich enough between shifts to cause an afterfire, you would probably be hearing the signature "pshhh" of a BOV. Check to make sure you don't have an aftermarket BOV installed. If so, replace it with a stock BPV. If not, check to make sure you don't have any leaks between the BPV and the turbo inlet. Also, check for other boost leaks... that could be causing you to run rich whenever you get into boost, which might explain why it happens only during WOT runs.
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    Registered User Julio Bro!'s Avatar
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    That's very interesting subypete, in fact, since I installed the cold intake, I do hear the "pshh" when shifting at over about 3500 rpms. But, I haven't done anything with the BPV, and I was told the sound was normal because of not having the OEM airbox to quiet it down.

    I could take it to the shop and check for a leak, but seems unlikely since this happened when the car was new. Shouldn't a boost leak throw CEL?

    I should point out, the bang occurs doing a first WOT, then it doesn't anymore, like the engine adjusts and it's OK. But it always surprises me, because this happens a couple of times in the year. Could it be caused by low quality fuel?

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    Registered User subypete's Avatar
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    If you're referring to 91 octane as "low quality" then no, it probably wouldn't be the cause of your problem. Low octane fuel is more likely to pre-detonate, increasing the likelihood of knocking. If it really was bad/expired/impure fuel, I suspect you would see other issues such as hesitation, poor idle, reduced power, etc. The fact that it only occurs between shifts really points to a boost leak though. Yes, if you have a CAI you will be more likely to hear the "pshh" sound on throttle lift, making it harder to determine whether or not you have a leak. Any good subaru dealer or mechanic will be able to pressure test your intake system and determine whether or not you have a leak. But before that, I'd pop the hood and look around for loose/unconnected hoses. You could also build your own leak tester:
    Boost leak test instructions
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    Registered User wreckingball man's Avatar
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    So this is a regular thing or no?

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    Pro Manscaper Mikie13's Avatar
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    My stock 07 STI with a simple Helix CBE afterfired like crazy. Then I went Cobb Stage 2 on it...began shooting flames during shifts. Then I got protuned, perfect AFRs...I could shoot 7 foot flames on command. Pretty damn cool if you ask me. I had no leaks at all, and the car ran perfectly fine. Some are just more prone to it than others.
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    Registered User pete25r's Avatar
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    normal backfire/afterfires happens when you have a vent to atmos. blow off valve.

    what the valve does that makes the backfire happen is it lets out metered air so then there is less air to mix with the fuel and it goes boom in/on your cat.

    you could have an air intake leak somewhere. check all of your vacuum lines on the air intake. shoot check all the ones you can.
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    Registered User anpato's Avatar
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    Quote Originally Posted by Julio Bro! View Post
    That's very interesting subypete, in fact, since I installed the cold intake, I do hear the "pshh" when shifting at over about 3500 rpms. But, I haven't done anything with the BPV, and I was told the sound was normal because of not having the OEM airbox to quiet it down.

    I could take it to the shop and check for a leak, but seems unlikely since this happened when the car was new. Shouldn't a boost leak throw CEL?

    I should point out, the bang occurs doing a first WOT, then it doesn't anymore, like the engine adjusts and it's OK. But it always surprises me, because this happens a couple of times in the year. Could it be caused by low quality fuel?
    Check the intercooler to throttle body hose, my car used to shoot flames from 1-2 on WOT because it came loose.
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