Small engine question
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This is a discussion on Small engine question within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; I've always heard people say that to check if your pistons are toast, you should open the oil filler cap ...

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    Registered User superdonkey23's Avatar
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    Small engine question

    I've always heard people say that to check if your pistons are toast, you should open the oil filler cap while the car is running and if there is raw exhaust fumes blowing from it than it is time for a rebuild. Well out of curiosity i decided to check it on mine. Problem was, it was blowing out something, I couldn't tell it was just normal air form the engine operating or exhaust. Can someone clarify this for me?

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    OMG WTF MainFrame's Avatar
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    The right way to check is to do a compression test.. and if you want to be more through and check for valve problems too do a leak down test as well.

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    Registered User superdonkey23's Avatar
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    Well i really don't want to do a leak down test for something i really don't think i have a problem with. I'm just asking what are the very clear signs of raw exhaust fumes. I smelled my exhaust and then smelled hat was coming out of the oil filler port. Both smelled completely different.

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    Moderator Donkey's Avatar
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    Quote Originally Posted by superdonkey23 View Post
    . Problem was, it was blowing out something, I couldn't tell it was just normal air form the engine operating or exhaust. Can someone clarify this for me?
    How do you expect us to clarify it over the Internet? Unless I have missed something you can't smell over the Internet.




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    OMG WTF MainFrame's Avatar
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    Quote Originally Posted by superdonkey23 View Post
    Well i really don't want to do a leak down test for something i really don't think i have a problem with. I'm just asking what are the very clear signs of raw exhaust fumes. I smelled my exhaust and then smelled hat was coming out of the oil filler port. Both smelled completely different.


    There's going to be some pressure in the crank case, whether the pistons/rings are good or not. Air coming out of the oil filler cap is not a good way to determine health of a motor, a compression test is needed for that.

    Now if there's smoke coming out of the oil filler, there might be a problem.. but you would still need to perform a compression test to determine what's going on.

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    Registered User Ingo's Avatar
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    My motorbike actually pull a vacuum and it can be clearly felt when you cover the hole with hand. It also idles real sick with the filler cap open. Haven't tried that with the subie yet. But (pretty much) all cars have a positive crank vent: a one-way valve connecting the inside space of the engine to the vacuum of the engine breathing side. There is ALWAYS some blow-by of combustion gases into the crank case, as mentioned above. If that blow-by can not escape somewhere - the engine would blow all seals and what-not. The motor oil in a running engine will make for a oil-fog atmosphere inside the crank case and that will cause it to smell whatever. If you get straight exhaust from the oil filler cap, that engine is already not running right and busted to hell. Forget about this whole thing, the only thing you may be able to tell from there is if you crank venting is working OK...
    You can't have everything - where would you put it?

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