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Re: left off oil filler cap- damage? (archive)
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Posted by Shawn Fogg on April 15, 2001 at 20:54:05:

In Reply to: left off oil filler cap- damage? posted by jiM\\\ on April 15, 2001 at 20:48:43:

" why does engine run rough with filler cap off...and did i do any permanent damage? "

Assuming nothing fell into the engine you should be OK. May want to think about changing the oil+filter... just in case.

The engine runs rough with the oil filler cap off because you are letting unmetered air into your intake. Same as a vacuum leak. The unmetered air gets into your intake through the PCV.

Normally the gases that pass through the PCV are blow by from your pistons. That air has already burned and it is small amounts anyway so it doesn't change your engines mixture in a meaningful way. With the oil cap off you are letting unburned air (oxygen) into the engine which gets into the intake and screws up your mixture.

Shawn

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