I was camping in the desert with my generator (onan microquiet 4000 with 224 hours)when it suddenly died. I have a toyhauler and it was about 90 degrees and it wasn't under a large load. So i opened the compartment everything appeared to be fine. I figured I would check the air filter, so I cracked the filter housing wing nut and bam out pours some oil.? all told about 1/4 to 1/2 cup came out. the filter had oil on it, not a lot but some. cleaned it all up and it ran fine. Why was there so much oil in the filter housing? any ideas? There is a breather tube from the valve cover to the air filter housing but is it supposed to put that much oil out? any ideas? Thanks in advance

Was it overfilled by chance? Has it been serviced lately? No, it should not be putting oil in there.

Sounds odd to me as well. Our Onan 4K unit has 800 hours and has run flawlessly for the 8 years we have owned our TH. I do all the maintenance and have never seen that much oil in the air filter. Since this Onan does not have an oil filter and utilizes splash lubrication maybe there was too much oil and some just happen to get into air filter. Glad you got it working and it's back to running fine. We've used our Onan during hotter weather than you stated powering entire rig including AC and she purrs along happily for many hours without issue. Great feeling knowing we can power our entire rig when boondocking with a simple press of a button.

Not sure about your onan but some air filters are treated with oil even the honda 2000I's soak the air filter in oil then squeeze out thethe excess oil. Did you have it serviced and maybe they got a little carried away with the oil? I know the K&N air filters also get treated with oil. Check your owners manual for the onan.

Devocamper wrote:Not sure about your onan but some air filters are treated with oil even the honda 2000I's soak the air filter in oil then squeeze out thethe excess oil. Did you have it serviced and maybe they got a little carried away with the oil? I know the K&N air filters also get treated with oil. Check your owners manual for the onan.I use to work as a mechanic back in the 70s and many of the engines used oil bath air cleaners. I know that K&N use to use an oil bath system on their air cleaners. It's very efficient at filtering fine dust.

the oil level is right in the middle of full and add. i don't oil the filter it's supposed to be dry(my understanding). I have done regular maintenance on it. I'm baffeled

oil in air filter for generator | Good Sam Community

17 Replies

Generator_Jim
Explorer
Feb 22, 2015
The internal engine system tries to run on a slight vacuum. If any gasket on the engine leaks air into the engine, it will instantly go to a pressure system and blow saturated oil vapor into the valve cover area which will overload the breather valve and pass oil into the air filter. Paper air filters that are saturated with oil will NOT pass sufficient air and the engine will run like it has the choke on.

JimL
GATJcampers
Explorer
Feb 22, 2015
the muffler is all clear.
nevadanick
Explorer
Feb 21, 2015
Did you check the muffler ?
azrving
Explorer
Feb 21, 2015
It's supposed to be flimsy. It acts as a one way valve. Be sure the little hole is open and be sure to put it back in correctly
GATJcampers
Explorer
Feb 21, 2015

here are pictures of my "valve". cannot tell if it's bad or not. One thing very noticeable the piece that looks like a spark plug gapper is super flimsy. I can see that letting everything through. Thoughts?
GATJcampers
Explorer
Feb 19, 2015
you guys are awesome. I will check these things out right away.
westend
Explorer
Feb 19, 2015
An off-level condition and a full crankcase can sometimes splash oil into the breather tube and it will then get onto the air filter. A 1/4-1/2 cup of oil in the filter canister is extreme. I would guess that you have a failed reed plate or breather device on the crankcase and the engine is pulling the oil directly out of the crankcase. If you didn't live in CA, you could just disconnect the breather from the filter canister (just kidding). On second thought, you actually could do this to observe the situation and see what results.
Dave_H_M
Explorer II
Feb 19, 2015
I think the history leading up to this situation is key.
azrving
Explorer
Feb 19, 2015
Here is a picture of a Briggs and Stratton breather valve which is the same concept on most small engines. The large hole is the "reed" or one way valve disc. The small hole and those small slashed spots in the gasket are the oil return hole that lets any oil/mist oil drain back down into the crankcase. The larger hole at the top right edge mates up with a tube that runs over to the air filter housing. If that little hole was to become plugged, it may fill with oil and allow it to enter the pipe to the air filter.

On page 6-13 of this onan manual
they talk about the breather but dont mention the drain back hole or show it. It would be a small hole in the baffle plate.

One Onan breather style
azrving
Explorer
Feb 19, 2015
Chris Bryant wrote:
I was waiting for one of the generator guys to chime in, but I'll swear there is a reed valve in there somewhere in the crankcase breather that can get stuck.
I could well be wrong though- that's not my forte...

Yes, the crankcase breather is often a flutter type "reed" plate and it could be his problem.

It let's the crankcase get rid of any blowby and it's directed to the air cleaner housing so it doesn't go out to the atmosphere. It's actually an emission control device as compared to the old small engines that had the little breather hose hanging off the side of the valve chamber cover or the draft tubes that hung down off car and truck engines.

If you had a ton of blow by it could push excessive oil but the op didn't mention poor performance or smoking bad so that's not the problem.

This manual may be close to your model but dont take it as gospel as I dont know the actual model numbers off your engine/gen.

It shows a breather set up on page 8.8 and 10 .2

Manual

before I tore it apart I would search the manual or other resources to try to find out what a manometer reading would be for that engine. The manometer would measure the vac or slight negative pressure in the crankcase. If the flutter/reed valve is stuck and not working properly you may not get a correct manometer reading and possibly push oil into the hose and filter area.

If the reed valve was to stick closed and not vent the crankcase, a positive pressure would build in the crankcase because of normal amounts of blowby and start pushing oil out of the engine where ever it can. Sometimes past crankshaft seals and sump pan gaskets and rings.

This video explains some of it. it's not the best but....

Crankcase venting

OP, has anything been done lately? Has anything different happened?

The last small engine I worked on that had a problem like that was a Honda 4 stroke walk behind snow blower. The last guy to work on it had installed the crankcase breather hose onto the wrong plastic fitting on the air filter housing. This created a build up of pressure in the crankcase and it started leaking like crazy around the crankshaft seal and sump cover gasket.

The one before that was a Kawasaki vertical shaft engine on a new JD lawn tractor. It was still smoking after being torn apart and rerung TWICE. I wasn't working on it but being one of those curious jobs to everyone in the shop we were all looking at it. The valve cover was off so I used a long screwdriver and put in down the passage way in the head and block that leads to the valve chamber breather area. The head was drilled for it but the block had never been drilled at the factory. It doesn't happen often with small engines but crankcase ventilation problems do happen.

So with all that said you need to know what's going on with the crankcase/breather. Maybe someone else has an answer. There is always something else that may be the problem.