The DRZ oil system is… busy

Which brings me to my first issue with my DRZ. A little while back I noticed a few ounces of oil coming out of the air box–puking out onto the swing arm–when I would park the bike after a short ride. “That ain’t right,” I said to myself.

I did some searching on the Internet and didn’t find a solid explanation or solution. So I cracked a beer and stared at the bike for a good long while. I poked at some of the hoses, traced their routes with my fingers, and found what I thought to be the source of the problem: I ran out of beer. Kidding.

The oil system in the DRZ400 is pretty dang busy. When the bike isn’t running the oil sits in the frame. When it is running the oil is pumped into the engine, out the top, around the side, into a black box, and recycled back into the bottom of the engine. This process repeats over and over until you shut the bike off. Heck, just to change the oil you have 2 drain plugs, a filter, and a plug with a screened debris catch. Needless to say, oil changes are a pain in the mass, which is like a big thick you know what 🙂 Back to my problem.

After betting I was right, I pointed my attention to this little black box. Its primary function is to catch the oil/air mixture coming off the top of the engine (crankcase) and separate the mixture: send the air into the air box, and put the oil back into the lower part of the engine. Pretty much the only way oil can get into the air box is through this black box.

It’s surgery time. Cool!

First I had to pull off the seat, three hoses, two side panels, oh, and the exhaust. Because the black box, though clearly visible from the exterior of the bike, wouldn’t come out any other way. The DRZ continues to amaze: all of this took +-10 minutes. Loves the DRZ, he does. Say hello to my little friend the “crankcase breather box”.


Oh yeah, it was dripping with oil. If oil had been successfully going back to the engine, this should have been a little cleaner. The replacement box I ordered was lighter by a good bit. Moreover, I found this little black box was also the source of a very incorrect (low) oil level reading. Because of the blockage this thing was creating, oil was not getting back into the frame once the bike was shut off, which coincidentally is when you’re supposed to check the oil level for this bike. This bike only holds 2 quarts of oil. If you think you’re low and put oil in when you didn’t need it you now have a new even bigger pain in the mass.

Fortunately I changed the black box out first, ran the bike for a few minutes, and then checked the oil. It was only a touch low. I had lost some oil from the original problem but not much. Whew! Time for a test ride.


Aww yeah! Problem solved. DRZ is back on its feet and the oil system is happy. This problem likely originated from the bike sitting too long in the garage of the original owner. Though the bike was immaculate, oil was gumming up in that black box and creating a blockage I wouldn’t encounter until a few hundred miles later. See… it’s just a machine, folks. A self-contained system of mechanical parts. Don’t be scared! All of the information to solve its problems is right in front of you. Patience and a desire to learn often lead to success. *thumbs up*


  1. Thanks for posting this ! Althought the oil level in the engine was fine and there was no leakes, it didn’t show the correct level of the oil on the dip stick. Looks like this is the solution.


  2. Sorry to say this but I tore the whole oil system off my drz and that black box doesn’t have oil pumped into it. It is a breather for the journal your can chain runs up to the head and also a breather off the valve cover. It feed to the airbox due to some emmisions bs that uses the warm air for combustion


    1. You’re right, when the system is performing optimally, there should only ever be a tiny bit of oil in there that gets skimmed off the top of the crankcase. And if the breather box is working right it should pass that small amount of oil back into the system. However, if you let the bike sit too long, or too much oil gets dumped into the breather box, it can get gummed up and clog the breather box. Then, anything that gets passed off the crankcase goes over the top of the breather box and straight into the air box and out the bottom of the air box. What you’ll end up with then is oil on your swingarm, which if it’s enough can be potentially dangerous if it drips down in front of the wheel.

      It will also lead to inaccurate or low oil readings as Patryk says above, because some of the oil system is clogged, and some of the oil leaves the system entirely through the air box nipple at the bottom.


  3. I’m struggling with the black box now I do know where the hoses go. It’s obvious where the two belong but there is one on top that I’m not sure about. Can you help me out


    1. The two hoses on the top: one runs to the top of the cylinder head/crank case (forward) and that’s a fatter hose, and the other is a thinner hose that snakes back to the air box (backward).


  4. great post, im also chasing down a low oil reading (after i measured 1.7L going in), pulled the box out today, that airbox hose clip is a bit of a…

    I’m not sure what this unit should weigh, but im pretty sure the breather line shouldnt be filled with honeycomb lookin stuff?

    hopefully will be back to getting good oil readings when i get it all back together


    1. I should have weighed the units back when I had one seeping oil and a brand new one. Great idea! Why didn’t I think of that??

      That box is full of honeycomb stuff unfortunately. Not sure if it moves around in there, or breaks apart and shift around. I wonder…

      Hope everything worked well after the fix. 🙂


  5. Its a breather box.

    My understanding is if oil is coming out of it then it is because of blow back….

    Perhaps having the breather box there keeps the engine running longer when there is blowpack from worn cylinder/rings… as instead of oil spewing out it has to get out through the top tube….

    I am however wondering what my issue is as after two months after a rebuild i have lost compression and spewig oil out the top breather…..


    1. A lot of motorcycle engines have a breather of some kind to account for the oil that gets skimmed off the top of the crankcase. (California specifically has a bit of an extreme method of dealing with this.) The E models of the DRZ 400 often just use a tube that leads to a small filter, or just dump it out the bottom along with any gas overflow. Again, it’s usually a really small amount.

      If you’ve lost compression and you’re getting too much oil out of the top, it could be a number of things. What oil are you using? Is the oil weight right? How about the valve clearances after rebuild, are you sure they’re set to spec?


      1. a few months have passed and a rebuild completed, but,,,, worked well for a week then popped a countershaft sprocket seal…… rode again,,,,, five minutes into ride spurterd oil everywhere…. .. ride again and it spurts oil straight away,,,,,,, so me thinks its a blockage,,,,, so check the vent holes again to the breather box, and no they are all clear….. So I think its a slipped ring on the rebuild,,,,, well take the head off again,,, no its perfect……….. Okay now I am down to blocked passages somewhere…. I have checked all passages which seem clear…. the only one that does seem a bit blocked is the one the oil pipe goes into under the stator side…………. I assume this feeds back to the oil pump on the other side???????? That’s an assumption,,, with real effort I can blow air down it……. Irrespective something strange is going on,,,,, the valves were in spec. I should have tested for compression,,,,,,, before it was like plenty of power but now there is none…… I am currently confused,,,,,, but no wait,,, yeah when i drained it I got 3 litres of oil,,,,,, so that might be it….. but then I think how can I measure to the fill mark and it never shows to the fill mark……….. Okay so heres the next question,,,, I think your wrong on your idea of how the system circulates oil,,, the question is does the oil go down the side pipe or up it, my thoughts are it circulates up the oil pipe then down the stem,,,, but I cannot see any discussion of how it circulates…


  6. Goodmorning. Im having a serious problem with my 2008 drz400sm. I had a stroker crank installed, big bore kit and stage 2 hot cams installed with the carb mod. Starts and runs good but at high speeds oil starts dumping out of the breather box breather line ( alot of oil ). Im guessing with the changes to the motor it now has ver hight compression. How can i fix this. Help


  7. The black box is part of the engine breather system and is there to try and separate some of the oil drops out of the engine pressure. The oil drops collect on the wire mesh and drip back down into the sump. Excess pressure is released into air box. Leaving the bike sitting for extended periods will not cause this box to fill with oil. If an engine is healthy then most times that laods of oil is found in the black box is due to overfilling the engine with oil. Oil checks must be done just after running the bike for 3 mins or so, as it is a dry sump system


  8. Unfortunately I did not know about the proper oil check procedure. I did refill the oil a couple of time until I read full of on the mark. I did had couple trouble after that like blue smoke etc. Now I have my whole engine open on my bench. Nothing appears wrong with it…… I wish I read this thread before 😉 Thanks


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