2015 yz450f crank case vent

5 replies to this topic
  • tgill1515

Posted November 08, 2015 - 12:14 PM


I have a 2015 yz450f that i just bought brand new off of the showroom floor. Im worried now because after last weeks ride it was pissing steam out of the vent tube on top of the valve cover while doing a fairly steep hill climb. After this happened i started to pay closer attention to the tube and i noticed that at idle it blows alot of air out of it . The pressure decreases as i get on the throttle. Is this normal or is the bike fudged already. Thank you for the help.

  • SAthump

Posted November 08, 2015 - 04:57 PM


Normal The pulses just get so fast at rpm that the amount of air moving decreases

  • tgill1515

Posted November 08, 2015 - 05:28 PM


Apppreciate the fast response. Thanks for the info. This is my first yz that ive owned so i was worried

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  • Krannie McKranface

Posted November 08, 2015 - 06:53 PM


You can put an automotive vent filter on the end of it, and it will still breath, but it will stop 'puking'....

  • grayracer513

Posted November 09, 2015 - 07:38 AM


Assuming that it is the 12mm hose running down from the cam cover, that is the crankcase vent hose.  The piston in your engine has two sides, the top, where all the good stuff goes on, and the underside that lives in the crankcase.  If you had at least two cylinders, and teh crank was built to have one going up while the other was going down, you wouldn't see much of this, but you only have the one, and it displaces 450cc of air every down stroke, and then wants it back on every up stroke.  Because there is more time between strokes at idle, the "pulsing" is more noticeable at idle then when the engine is running faster.  The air pressure within the crankcase will rise and fall, but air is compressible, and because it takes a certain amount of time for it to start moving one way or other, and the engine is moving so fast, the pulses seem to vanish.


The main function of the breather is to prevent pressure from building in the crankcase as the temperature rises, the oil starts releasing gasses, and most importantly, combustion gasses leak into the crankcase past the rings, which last is referred to as "blow by".  This is where the slightly outward net air flow under throttle comes from, and to a degree, it's normal, because even healthy rings leak "some".


The oil it drools is normally nothing more than accumulated vapors being blown out the hose by all the activity, and will increase the harder you run it.  Expect some, but remember that overfilling the engine will cause it to blow out an excessive amount, too. 

  • tgill1515

Posted November 09, 2015 - 08:11 AM


I appreciate the responses from everyone. Grayracer513 , yes i over filled the oil the first time i changed it, forgot to crack the bolt on the fill sode to let all the over filled oil to spew out. Im not having any oil come out of the hose. Atleast not to my knowledge or not that i have seen , just alot of air. I was worried because it really spewed out the air at a high pressure about half way up a hill climb so i killed the bike immidietly, i thought i blew the headgasket i was worried sick. Put er in nuetral and coasted down the hill to my truck, i double checked my radiator fluid when i got home cause at first i thought i overheated it, i knew the 450's ran hot but i wasnt thinkin poppin headgaskets hot hahah. So after i got home i fired her up and noticed all the air underneath and followed the hose up to the top of the valve cover, makes sense to me now, just my first yz450f. Actually first brand new bike as i came off of a 05 ltz400

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