Crankcase Pressure

3 replies to this topic
  • Hammdy

Posted May 13, 2016 - 09:36 PM




This is my first post here but I used to be on the forum many years ago...just couldn't remember my user name, email password, etc...anyways....  I just bought a used 2011 yz450f, only test drove it up and down the street at the guys house and didn't notice anything irregular.  When I got it home I ran the crankcase breather tube to a catch can located along the back of the main frame on the left side near the rear shock.  Took it to the track and after riding hard for about 15 minutes or so it was spewing oil everywhere.  Took it home and the output shaft seal had popped out.  I figured out that the breather tube was kinked underneath the tank not allowing it to vent.  I rerouted it and anchored the tube so it wouldn't get kinked anymore.  Cleaned up the output shaft seal put it back in and started it.  I pulled off the oil filler cap to top off the oil as it was running and noticed a lot of pressure blowing out the filler which seemed unusual to me, but I don't know these engines too well.  Before I could top it off it was spewing oil again from the output shaft seal.  Decided to do a leakdown test to see if there's a head gasket or rings problem.  It was leaking a lot from intake and exhaust valves (only held 15%!) but didn't notice crankcase leakage.  So down to the questions....  Should there be a lot of pressure in the crankcase when it is running?  Wondering if the seal just popped out the second time because it was reused.  Horrible leakdown test - Was it just so bad because the engine was cold?  It seemed to pull hard when I rode it.  Kind of lost here with this bike....I thought I bought a good condition bike.

  • grayracer513

Posted May 14, 2016 - 07:00 AM


First, the under side of the piston also displaces 450cc of air, same as the top side does, so at lower speeds, you'll normally see air pulsing in and out the breather, or an open oil cap, quite vigorously to match the up and down stroke of the crank.  Then there will always be at least a small amount of combustion gas that leaks past the rings to create a net outflow from the engine.  If the breather is blocked, it will push something out of the way somewhere sooner or later.

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  • Hammdy

Posted May 14, 2016 - 08:37 AM


Well I think I got to the bottom of the problem.... Went to the dealer to pick up a new seal and noticed on the microfiche that there's supposed to be a plate bolted to the case to hold the seal is missing  :foul: Not sure about the poor leakdown though.

  • grayracer513

Posted May 14, 2016 - 09:17 AM


The plate isn't intended to hold the seal in, it's intended to keep mud and sticks off the seal.  It will help retain it, but with your breather pinched off, it would have then likely pushed out one of the "half moons" on the cam box cover gasket.

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