Engine Blow By ...

[DAP]

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]

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]

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]

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]

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]

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]

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]

</font><font color="blue" class="small">( 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. )</font>

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]

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]

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.