Onan P218G Crankcase Breather Valve Issues

[CrossThread Industries 31]

Hi Everybody, I found a good deal on an almost unused 518H.

Onan P218G engine. Immaculate.

Question 1) The Hobbs indicates 900 hours and there is no way this unit has seen more than 50 active hours.

Did I read somewhere that as long as the key sits in the ignition, the Hobbs runs?

 

Question 2) more serious, after 10 minutes of running from cold, a ton of oil is making its way up through the crankcase breather tube into the carb and belching a huge white cloud of smoke.

It burns off, then repeats. The previous owner mentioned this problem.

I read every manual I could find but I don't see how that much oil can get purged up into the air filter area. Is there a plugged oil return path? Perhaps it has something to do with the reed valve not work correctly, maybe.

I found a used one on ebay (pic below) but I don't know if replacing the reed valve group really cover what's broken about this setup.

Does anyone have any experience with this ?

[Achto 27,570]

3 hours ago, CrossThread Industries said:

Did I read somewhere that as long as the key sits in the ignition, the Hobbs runs?

 

The key has to be in the RUN position in order for the Hobbs hour meter to run.

 

3 hours ago, CrossThread Industries said:

I could find but I don't see how that much oil can get purged up into the air filter area

 

Worst case - If you have bad piston rings this will cause excess engine case pressure and force oil out of the breather.

[WHX?? 48,815]

If it sat for awhile stuck rings?

[lynnmor 7,305]

Most Onans have the hour meter wired thru the oil pressure switch so that the meter will not run, apparently the 518-H isn't wired that way.

 

The breather tube should be a tight fit going thru the bottom or the air filter housing, that loose fit allows dirt into the engine.

 

The breather may not be assembled correctly.

 

[CrossThread Industries 31]

1 hour ago, Achto said:

 

The key has to be in the RUN position in order for the Hobbs hour meter to run.

 

 

Worst case - If you have bad piston rings this will cause excess engine case pressure and force oil out of the breather.

I have to look at one more thing I thought of after I did my 10 minute test.

It seemed like the engine would burn oil after 10 minutes, burn it all off over about 10 to 15 seconds and then recover.

What I think is happening is while I am driving around on uneven ground the oil pools up in the air cleaner area and under the right conditions pours into the carb to get that burn off.

So, the oil loading up may be continual but the burn off only happens when I get a little slope going and the pooled oil has a chance to run into the carb.

WHY it is pushing oil could be your ring theory - I am not too worried if this is all it is because the engine has only seen a little action.

All the bolts are clean, no rust - the tractor was well sheltered.

[CrossThread Industries 31]

1 hour ago, WHX?? said:

If it sat for awhile stuck rings?

Thanks for the help - something wrong with the rings seems to be developing as the cause of the oil purge - will get a rebuild kit, if so.

[CrossThread Industries 31]

1 hour ago, lynnmor said:

Most Onans have the hour meter wired thru the oil pressure switch so that the meter will not run, apparently the 518-H isn't wired that way.

 

The breather tube should be a tight fit going thru the bottom or the air filter housing, that loose fit allows dirt into the engine.

 

The breather may not be assembled correctly.

 

 

Excellent info, thanks for the drawing... I will start by disassebling the breather circuit and checking the reed valve.

Definitely getting the feeling it's the rings.

[squonk 41,079]

518H hour meter circuit 88-89

 

[CrossThread Industries 31]

29 minutes ago, squonk said:

518H hour meter circuit 88-89

 

 

Thanks - I will check to see if any electrical got changed - there does seem to be a home-job on a switch for the lights. You never know what happens when the owner tinkers.

Cheers - PJ

[Wild Bill 633 845]

I am not familiar with an Onan, but when a Kohler Command blows a head gasket between the cylinder bore and the oil drain back hole, the compression can push oil past the breather.

[lynnmor 7,305]

35 minutes ago, Wild Bill 633 said:

I am not familiar with an Onan, but when a Kohler Command blows a head gasket between the cylinder bore and the oil drain back hole, the compression can push oil past the breather.

That can only happen on an overhead valve engine.  Other than the breather or the block being knackered, only real bad rings or really bad valve guides would cause the condition in question.

[RED-Z06 2,221]

Oil looks discolored...possibly fuel thinned?

Edited May 13, 2023 by RED-Z06

[Wild Bill 633 845]

Again, I am not familiar with an Onan, but, in the picture above of the air cleaner housing, there is a hose coming up thru a hole which is not totally sealed. If the hole is not sealed, dirt can bypass the air cleaner and get sucked into the engine causing damage.

[lynnmor 7,305]

5 hours ago, Wild Bill 633 said:

Again, I am not familiar with an Onan, but, in the picture above of the air cleaner housing, there is a hose coming up thru a hole which is not totally sealed. If the hole is not sealed, dirt can bypass the air cleaner and get sucked into the engine causing damage.

Apparently he has the earlier version of the breather tube, they are getting hard to find and he should buy two if he intends to keep the tractor.