I just picked up a 2017 zForce 800 Trail and this is my first CFMoto.  Whenever I go for a ride I have gear oil leaking from the rear diff vent tube. 

I drained the rear diff and refilled with exactly 250ml of 80w90 gear oil (per the manual).  I put on a new vent tube, went for a ride and I am still getting fluid leaking.  I did a quick 6 mile loop with speeds constantly between 30 and 45mph.

I'm at a loss as to why this is happening.  I understand if the diff was overfilled or had water in it, but neither is the case.

What am I missing?  Do zForce just puke gear oil or what?  I something else wrong that I'm not thinking of? 

I'm starting to second guess my purchase.

my rear diff did the same i honestly was at a loss an just let it puke cause if its puking its still full lol
then switched to lucas synthetic 75/90 rather an haven't had an issue not sure if the 80/90 foamed up or what but haven't gone back since an run it in front rear an trans now 

[2015 Z Force 800 53/Trail EPS]

Mine is also at the right level and it will send a little up and out the tube when...and only when I have been running it above 30mph on forest roads for extended periods. Interestingly, this is also when my belt temps get their highest and per my IR heat gun, the rear diff is over 225 degrees..which is also the hottest it gets. I will say it has slowed noticeably after I switched from 75/90 synthetic to 90w140 conventional because of all the grinding going on. My git tells me the vent port must be right over the ring gear and is hit with slinging lube all the time. If I find that's true when I pull it down I will be re-locating it away from it. In the mean time I suspect a catch-can system on the tube may help. It just has to be an expanded area to collect it and let it run back down...like a gutted inline fuel filter.

Thank you both for the feedback.  I'm going to take the advice from both of you.

I'll change the fluid to Lucas Synthetic 75w90 and I'll install a "reusable" motorcycle fuel filter with the filter removed.  This should allow for about 2-3oz of fluid to be collected and returned to the diff before it starts puking out the other end of the vent tube.

https://www.amazon.com/gp/product/B0753F6PF4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

[2015 Z Force 800 53/Trail EPS]

Thank you both for the feedback.  I'm going to take the advice from both of you.

I'll change the fluid to Lucas Synthetic 75w90 and I'll install a "reusable" motorcycle fuel filter with the filter removed.  This should allow for about 2-3oz of fluid to be collected and returned to the diff before it starts puking out the other end of the vent tube.

https://www.amazon.com/gp/product/B0753F6PF4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Perfect. Think I'll get one too.

I installed the fuel filter "catch can" this afternoon and am happy to report that I no longer have any gear oil leaking.  More testing is needed, but the initial results are very good.

[2015 Z Force 800 53/Trail EPS]

I installed the fuel filter "catch can" this afternoon and am happy to report that I no longer have any gear oil leaking.  More testing is needed, but the initial results are very good.

Good. I ordered one too. It should work fine. The concept is used on engine vent tubes all the time. It does work.

Will this work on the vent going into the air box. I'm haven an excessive amount of oil. Also I'm getting new tires can I put 26-11's on the front. Will I have any problems with 27-11's all around?

[2015 Z Force 800 53/Trail EPS]

Will this work on the vent going into the air box. I'm haven an excessive amount of oil. Also I'm getting new tires can I put 26-11's on the front. Will I have any problems with 27-11's all around?

It's too small for a crank case vent. There are larger catch cans that will work though. There was a recall on some of the 500s that had excessive oil travel up the crankcase vent tube as I recall. That meant add a baffle at the case. If you have excessive blowby though, that may indicate a problem with rings.

I would NOT put 11s on the front. Not even sure you can and not rub inside somewhere but it murder on steering and other front end parts. 10s...are probably the max and no problem with 27 all the way around. If you are using the stock wheels know that different width wheels on the same width tires will give you different diameters...and that's a no-no on all 4wd systems. Just can't have the fronts and the backs trying to turn at different speeds. Get you a good set of all the same 7 or 8 inch wide wheels and slap a set of 27x10s...or 9s.. on them and she'll be happy.

Sounds good. Thanks for all the good advice!!

[Late to party post] I just took the z500 out to gas 'er up and when finished I noticed an oil drip that I traced back to the rear diff vent tube.  Doing a search on the matter lead me to this post.  I was traveling on road speeds, so that part matches what others have said.  Also, after opening the fill port I could see the oil level right at the threads, so no filling was needed, suggesting unwanted additional volume due to moisture accumulation.  I'll give the Lucas 75/90 a try and I also have ordered a small filter to put inline with the vent tube as a line of defense.

[2015 Z Force 800 53/Trail EPS]

[Late to party post] I just took the z500 out to gas 'er up and when finished I noticed an oil drip that I traced back to the rear diff vent tube.  Doing a search on the matter lead me to this post.  I was traveling on road speeds, so that part matches what others have said.  Also, after opening the fill port I could see the oil level right at the threads, so no filling was needed, suggesting unwanted additional volume due to moisture accumulation.  I'll give the Lucas 75/90 a try and I also have ordered a small filter to put inline with the vent tube as a line of defense.

Yeah filling to the threads is too much for any CFM. Only put in what the book says...or do like I and many others, install a catchcan inline. It does work.

Swapped out existing fluids with 400R/330F mL of the Lucas 75/90 and added the catch can from a motorcycle fuel filter.  I found one that I could see into and open up to empty/clean if needed.  https://www.amazon.com/dp/B0BJ8NLMBS?psc=1&ref=ppx_yo2ov_dt_b_product_details  I removed the little filter basket before putting everything inline with the vent tube.