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pcv valve

mower17's Avatar

Join Date: Feb 2003
Posts: 365

02-23-03, 02:17 AM   #1  
pcv valve

I hear a lot of talk about a pcv valve, but I don't know what it is. I know briggs engines have a breather over the valves with a tube going back to the carb. Why does that tube go back to the carb? What type of breather do other companies use? Does the breather just blow air or does it suck in air too? As usual I am trying to learn small engines so teach me as much as you can. I do have a book on small engines and I read all 550 pages but still it doesn't cover EVERYTHING in detail. Thanks you so much for your time and patience.

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Join Date: Feb 1998
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02-23-03, 03:00 PM   #2  
Hello: mower17

The breather you're referring to is infact the pvc valve. Pcv means Positive Crankcase Ventilation. The device is there for several reasons.

One of them is to vent the oil fumes out.
Another is to allow for air expansion and contraction as the piston revolves up and down.
Another is to vent the fumes, gasses and or oil vapors into the carb so they can be burned. Burning them reduces air pollution.
Another reason to have a breather is to maintain or reduce pressures building up inside the crankcase.

If pressure where to buildup, the downwards motion of the piston would be working against the pressure reducing the engines output power.

All engines require a breather and every engine, small or large used for any power purpose has a breather of some type. Years ago the breathers use to vent directly out into the air.

To reduce air pollution, the breathers are now vented into the carbs or intake manifolds. Either method works and both types are used depending upon manufacturer and engine model and type.

An engine without such a device would also cause carbon to buildup in the crankcase and or gums, vanishes and sludge. Simply because moisture is produced during the fuel burning process and during periods when the engine is not being used.

All that being the possible cases, ventilation is required. The pvc valve or breather device allows ventilation and air circulation inside the crankcase to reduce or eliminate any or all of the above potential engine problems.

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