Why is My Honda TRX400 Blowing Out Oil?

If your Honda TRX400 ATV keeps blowing out oil from the crankcase breather after an engine rebuild, could it be that you didn’t break the engine in properly?

Partzilla’s Honda ATV expert John Talley hosts live Q&A sessions on our YouTube channel every Friday at 3pm Eastern. In this session, John was asked what could be causing a Honda TRX400 to be spewing out oil from the crankcase breather after a top end rebuild.

Honda TRX400 blowing out oil

Honda TRX400 Blowing Oil from Crankcase Breather Q&A

Hi John. I have a TRX400EX blowing oil out of the crankcase breather. Replaced everything, including new top end with new bored out sleeve. I installed a new oil pump, and all screens and lines are also fine. Any thoughts?

John Talley: The reason that happens on every machine I’ve ever seen is when you’re getting too much blow-by going past the rings into the crankcase. … How did you do the break-in procedure, and did you do it long enough and aggressive enough when you did the rebuild? It should seal up very quickly. We’ve done videos on breaking in different machines … but it sounds like she hasn’t really sealed up all the way. I would recommend if you’re unsure of it, to go ahead and do a leakdown test, and that should tell you what’s going on. There’s no getting around a leak down test, it is the absolute truth or lie detector. So give that a shot and see what comes out. Beyond that, maybe just run it a little bit harder and longer and see if she seals up like she’s supposed to. Also, I can’t believe I didn’t think about this, but make sure you haven’t overfilled it, because it’s kind of a dry sump system with the engine and the oil in the bottom of it, and then you’ve got the oil tank as well, so just be very aware of checking the oil in it. We have a video on that too, which may have been the first video we ever did, from eight years ago? So take a look at that.