Explain Engine Vacuum/Pressure to Me

[leeave96 487]

I'm trying to understand engine pressure and vacuum and how this works in a small engine. I've got a crank case vent - which I assume allows crankcase pressure to vent out? But if I look at Kohler manuals, they say that if I check the engine pressure from the dip stick tube, I should read 4 or more inches of vacuum.

I'm confused. Is the engine crankcase pressurized or does it create a vacuum?

Reason I'm asking is I have a Kohler engine that is leaking oil at the crankcase seal. I would think that the engine crankcase breather would relieve this pressure, but it appears clear.

Any explanations would be much appreciated.

Thanks,

Bill

[300zx 7]

Bill, if you look at the crankcase vent, it will have a small reed valve (thin piece of flat spring metal) over the vent hole that allows air out, but not in so that it creates a vacuum when the piston goes up in compression stroke. When the piston comes down it relieves the vacuum. That is why on small Kohler engines, like the 8 hp, they use impulse fuel pumps. The pressure pulses on the fuel pump diaphragm pumps the fuel. If that reed valve is in backward so that you build pressure in the crankcase rather than venting it, you will have leakage.

I hope this helps,

John

[tunahead72 2,386]

Bill,

I think John's on the right track with his explanation.

Which particular engine are you looking at? There have been several threads in the last year or so that addressed what the vent assembly should properly look like, but the designs are slightly different from one model to another. If you have a service or parts manual for it, you should be able to find a good diagram of how it's all put together.

I don't have a lot of experience with these things, but I do know that replacing a couple of pieces of the breather on my K241, and installing it correctly, got rid of a minor leak around my governor shaft.

[leeave96 487]

This is for a Kohler Command horizontal shaft engine.

Thanks!

Bill

[Rooster 191]

All small engines have crank case "pulses" ...

When the piston goes up the cylinder, the volume inside of the crank case increases, as it comes down, the volume decreases. as John explained the reed valve vents allow air to be pushed out on the down stroke of the piston, but not in on the upstroke, causing a vacum inside the crank case.

As the rings and bore in your engine wear they will begin leaking , allowing more pressure into the crank case than what it can vent causing the amount of vacum to reduce and eventually they can actually start building pressure in the crank case. One of the signs of badly worn rings is oil being pushed out of the breather tube. This is caused by the excessive pressure in the crank case.