Engine Blow By ...

DAP · Jan 28, 2004

The only thing I know about 'blow by' is that it isn't desirable. I don't know what it is, what type of engines it can effect other than deisel if any or anything else about it.

Can someone demystify this for me? /forums/images/graemlins/ooo.gif

Thanks.

gsganzer · Jan 28, 2004

I believe you're referring to losing compression past worn rings in the piston/cylinder. You check this by doing a compression test on each cylinder. The manufacturer or dealer can tell you what the acceptable compression range should be.

normde2001 · Jan 28, 2004

That "blowby" is a major cause of pressure/lack of vacuum in the crankcase, causing oil to be forced out of seals and breather.

BillyP · Jan 28, 2004

As gsganzer stated, blow-by is caused by worn rings. When the engine fires, part of the gases, in the cylinder/s are blown by the worn rings. It'll cause the oil to turn black, quicker than usual. If it gets too bad, you won't have enough compression and have to have an overhaul.

jinman · Jan 28, 2004

As everyone has noted, excessive blow-by usually involves worn or damaged piston rings. On my small 18 & 16 HP Onan engines there is a designed blow-by. If you pull out the oil dipstick, it will blow oil all over you. /forums/images/graemlins/tongue.gif The reason Onan does this is they use the pulsating pressure to power the fuel pump. The fuel pump's diaphram has a tube that goes into the crankcase, and it pulsates and pumps with remarkable efficiency. /forums/images/graemlins/cool.gif

I guess this is probably "too much information," but I thought I just give an instance where blow-by is not bad.

I don't know how old you are, but we used to have cars with an engine crankcase vent tube. When one of these engines with bad rings was in front of you, you could see blow-by puffing out the vent tube. /forums/images/graemlins/smile.gif

JDgreen227 · Jan 28, 2004

Jinman,

Thanks for the informative post, I have the 18 hp Onan in my old JD 318 and was aware the oil cap needs to be tight to prevent oil leakage but never knew the real reason. Since you know more about that engine than I do, got a question.

I get some oil in the air filter housing, would you know if the oil is created by normal blow by or would you suspect the rings are worn? Compression is close to normal. Thanks.

Bill

gsganzer · Jan 28, 2004

jINMAN,
I had the same problem with an Onan BF 16. In my case the rings were too worn. However, there is a reed valve and mesh demister in the line you're talking about. They're there to prevent oil mist from entering the air filter. It might be that one or both of these is not right.

jinman · Jan 29, 2004

( I get some oil in the air filter housing, would you know if the oil is created by normal blow by or would you suspect the rings are worn? Compression is close to normal. Thanks. )

Another coincidence? On my 18 HP Onan, I had so much blow-by that the engine would smoke and the plugs foul. I didn't want to rebuild the engine because it seemed to start/run okay and I think the demister that GSGanzer mentioned is missing. Anyhow, I took the crankcase vent tube out of the breather and routed it to a 1-quart oil can to catch the oil instead of let it blow into the engine. What a difference! The engine quit smoking and since the intake vacuum was no longer felt on the breather tube, there is only a residue of oil in the container. The experiment worked so good, I just left it that way. /forums/images/graemlins/blush.gif /forums/images/graemlins/laugh.gif. Since the Onan is on my old Jacobsen 3-wheeler (attachment), I may fix it someday, but my bit of "southern engineering" eliminated the spark plug fouling and engine smoke.

I have an Onan service manual for the engine, so I'll check and see if I can find the demister (after I remember what I did with the manual /forums/images/graemlins/crazy.gif). Let me get back to you on that... /forums/images/graemlins/smile.gif

BIGUN · Jan 29, 2004

On some gasoline opposed twin cylinder engines where the both pistons move in and out at the same time you could have quite a bit of pressure build up in the crankcase when both pistons are on the down stroke. To prevent this, some manufacturers install a check valve on crankcase vent. This has the effect of pumping the air out of the crankcase to the point that the pressure is atmospheric at BDC and goes negative as the pistons move outward. This system relies on the crankcase being sealed everywhere including the dipstick. Running negative pressure on the crankcase most of the time helps prevent blow by of the seals and piston rings. My neighbor learned this the hard way once when he tore down his twin cylinder B&S looking for a broken ring after it started smoking something awful, only to later discover the smoking was cause by one of his grandkids leaving the dipstick lose. /forums/images/graemlins/smirk.gif

I have heard stories of shrimpers down on the coast over-revving the old diesels in their shrimp boats so high that sometimes they would start running on the oil blow by from the crankcase. At this point the only way to stop the engine before it self destructed was to seal off the air intake with a metal plate. /forums/images/graemlins/crazy.gif

JDgreen227 · Jan 29, 2004

LOL,

when you said "3-wheeler" my brain automatically kicked in with ATV, and "Jacobsen" made me think of my old orange Snoburst. Looked at your attachment and started laughing becuz it was so different than I had pictured. For what it's worth the plugs on my JD never have oil on them. No rush to provide the info, and thanks again.

higgy · Jan 29, 2004

the old gray marine and gm 2 cycles would do this once and awhile.

I have seen this happen with industrial air compressors as well, when a seal or ring is broken oil or vapor from the crank case gets into the compression chamber and it will diesel for a short while, until it comes apart.

It is truly a scary sight to behold.

TOMLESCOEQUIP · Jan 29, 2004

When I built my house in '93, I bought an old mustang skid-steer loader to assist me with back filling and landscaping. It had a wisconson V4 and smoked like a freight train (MORE than a freight train actually) out the breather. After running it about an hour it would slobber it's way through a quart of oil out the breather ( talk about real blow-by !!) My friends who saw it would laugh and remark about the excessive use of oil...............But in that hour I moved 13 tons of bank run for the approach to my barn............All for the cost of a $1 quart of wally world oil !! I told the friendly nay-sayers that I would gladly shut off the smoking mustang if they would spread and level the next load of 13 tons for the $1 I just spent on oil !! I eventually routed the breather hose into a old tasters choice instant coffee jar ( large hole in center for hose, small holes punched around outside of lid for vent action).........And then I could reuse about 2/3 quart of the spewed oil after an hour.(YES it was still on there when I sold it, NO I don't feel guilty !) I used that old skid loader for the entire summer and sold it at a slight profit when I was done using it . I don't think the old wisconsin had enough power left to break any major internal parts as long as the oil was checked periodically.This is probably the only good ending story about blow by you'll hear............Tom

jinman · Jan 30, 2004

( It had a wisconson V4 and smoked like a freight train (MORE than a freight train actually) out the breather. )

That sounds like my old Ford 4000 Industrial that smoked everywhere there was an opening: exhaust, intake, engine crankcase, and sometimes even transmission. /forums/images/graemlins/blush.gif It would start and run just fine after I put a 12 volt system on it and added anti-foul adapters to the spark plugs.

It had a very powerful FEL and I used that tractor to clean up a mobile home park after I closed it. One day a cop was cruising by and stopped while I was working. He jumped out of his car and came running my way, so I shut down the tractor and said, "What's wrong?" All of a sudden he stopped in his tracks and said, "Oh! Never mind. I just thought your tractor was on fire for a second and you didn't know it." /forums/images/graemlins/shocked.gif As soon as I shut down the engine the smoke had stopped. We both got a good laugh out of that. /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

The tractor was still that way when I sold it, but the new owner said he had a shop and would rebuild the engine immediately. /forums/images/graemlins/cool.gif

jinman · Jan 31, 2004

( No rush to provide the info, )

The attachment is a picture of the crankcase breather on an Onan gasoline engine. After looking at it, I think my reed valve just may be stuck. Anyhow, the specs say the compression for the 16 and 18 HP engines should be between 75 and 115 psi. If yours is smoking, it's probably either the rings or the crankcase breather letting too much crud into the carburetor. I have other specs from my Service Manual, so if you have any questions, maybe we should start a thread specifically about the Onan. Some other folks might want this info too. /forums/images/graemlins/smile.gif

Junkman · Jan 31, 2004

Here is a link to a Yahoo Onan Engine club that has a lot of knowledgeable members that might be able to help you with the specific information... It is like going to a tractor specific club here... specific knowledge vs. general knowledge.. web page

JDgreen227 · Jan 31, 2004

jinman,

Thank you for the info, if I had known better I would have purchased the shop manuals for my 318 back in '89, they were much cheaper then. These days, I am the guy who carries no cash.... /forums/images/graemlins/grin.gif

JDgreen227 · Jan 31, 2004

Junkman,

Thanks to you also. I belong to the Yahoo JD Garden Tractor group but was unaware of the Onan Engine Club.

Sigh....so many interesting websites, so little time to surf them. /forums/images/graemlins/frown.gif

Bill

Junkman · Jan 31, 2004

There are engine specific clubs for each brand. These clubs are moderated so you won't be getting any **** messages with your answers..
Tecumseh
Kohler
Briggs & Stratton
Onan