Why Is There So Much Oil in My Yamaha ATV’s Airbox?

What does it mean when a Yamaha ATV has excessive oil in the airbox? And also metal shavings in the oil? It’s not good news. Just ask our ATV expert John Talley.

John hosts live Q&A sessions on our YouTube channel every Friday at 3pm Eastern. In this session, John was asked why a Yamaha YFZ would have a quart of oil backed up into the airbox and metal shavings in the oil.

Yamaha ATV oil in the airbox

Yamaha ATV Oil in the Airbox and Metal Shavings Q&A

Hey John, what would cause a quart of oil to back up into the air box of my Yamaha YFZ? Also, when I drained what was left of the oil, a good amount of metal shavings were in the oil and oil filter. Any thoughts?

John Talley: Oh boy, your engine is in trouble. Usually when it spits that much oil back into the intake, that’s caused by a little bit too much blow-by on the piston rings. And X amount of blow-by is going to happen. You’ve got a venting system that goes back up into the airbox for the crankcase. And when that becomes too much, sometimes it spits oil back into it. Now, the other possibility is that sometimes people overfill it, because that one is a dry sump system, and there’s a particular procedure you need to follow when you’re checking the oil level in it. And I’m pretty sure we did that when we rebuilt our 09 Yamaha YFZ450R, so if you would, reference that particular video and I can walk you through it. If not, just the oil change, I may have done it in that video as well. As for the metal shavings, something’s going wrong in a bad way. Now what is it? I have no idea. You can determine if it’s transmission related, or if it’s piston or bearing related. And when I say bearings I mean the connecting rod bearings. Take a magnet, run it over the top of the shavings that you found, and if they run to the magnet, well that’s probably transmission. And if they don’t, well that tells me they’re aluminum, or brass, and that’s probably going to be the bearing and/or the piston side of the equation.