Occasional slight "bumps" in idle
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This is a discussion on Occasional slight "bumps" in idle within the STi Engine Performance forums, part of the STi Technical Forum category; Hey all, just got a 2012 STi a couple weeks ago and I've noticed a few "bumps" in the idle ...

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    Occasional slight "bumps" in idle

    Hey all, just got a 2012 STi a couple weeks ago and I've noticed a few "bumps" in the idle from time to time. Not sure if it's due to gas quality, break in or if there could be an issue. The RPMs don't jump around, there's just a few bumps in the idle every so often. It's not that noticeable but I'm the type that notices everything when it comes to a car. I doubt it's an issue but I just wanted to get some opinions on possible causes. I'll have the dealer check it out when I take it in for the free 7500 mile service if it doesn't clear up. Thanks guys and gals!

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    Registered User power091999's Avatar
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    I am not 100% sure if it's an issue but I will give my .02.

    I am on my 5th Subaru
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    STi
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    Legecy GT
    STi

    All of them have those occasional "bumps" at stop lights and general idling. I feel it may just be something the boxer engines do from time to time.

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    Admiral Ackbar the 1st mycologist's Avatar
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    Or your A/C is kicking on (will also happen on defrost which uses A/C)
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    I've noticed it with and without the A/C running but I'm glad to hear it's pretty common with the boxer. This is my first boxer so I'm slowly getting used to it. It's hard for me because I'm the type that hears every noise a car makes and the STi makes a ton...but most I've heard are normal from what I've read. Thanks for the replies!

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    Registered User power091999's Avatar
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    Going with with mycologist said its very common for the car to do a little shaking when the AC kicks on and off. Like you however I have experienced the "bump" with no AC/defrost running. Simply sitting at a stop light with the windows down listening to the engine noise.

    I have learned that it's nothing to worry about, but if your still concerned then have your dealer look at it anyway. Always better to be safe than sorry.

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    zax
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    Quote Originally Posted by mycologist View Post
    Or your A/C is kicking on (will also happen on defrost which uses A/C)
    This!!

    If your using your brakes as well. The brake booster will draw some parasitic loss. Try this sometime: pump your brakes at a stop light and you will feel the car stutter for a split second.
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    Registered User power091999's Avatar
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    Actually never considered the brake drawing.

    Ill look out for that just out of the sake of curiosity.

    Thank Zax.

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    Administrator Trainrex's Avatar
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    Quote Originally Posted by zax View Post
    This!!

    If your using your brakes as well. The brake booster will draw some parasitic loss. Try this sometime: pump your brakes at a stop light and you will feel the car stutter for a split second.
    That isn't parasitic loss. That is a sudden vacuum change and the IAC compensating for it.

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    zax
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    Quote Originally Posted by Trainrex View Post
    That isn't parasitic loss. That is a sudden vacuum change and the IAC compensating for it.
    Ha, good point. I definitely didn't word that correctly... it's not like the brake booster is connected directly to the crankshaft.
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    Quote Originally Posted by Trainrex View Post
    That isn't parasitic loss. That is a sudden vacuum change and the IAC compensating for it.
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    Quote Originally Posted by Mikie13 View Post
    pwned.
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    I'm not sure I can handle this talk of parasites and vacuums though...

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    Quote Originally Posted by ZackUSAF82 View Post
    I've noticed it with and without the A/C running but I'm glad to hear it's pretty common with the boxer. This is my first boxer so I'm slowly getting used to it. It's hard for me because I'm the type that hears every noise a car makes and the STi makes a ton...but most I've heard are normal from what I've read. Thanks for the replies!
    I hear it in my 2012 wrx too. Engine goes regular boxer UHL rumble and then slight miss (not a miss..but misiing a beat for a 1/10 second?)...only if you are really listning to the engine...

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