PCV - Whats it do?

[gmeddy]

Ok so the V must stand for Valve right?
so calling it a PCV Valve is a double up...

whats it do? restrict the amount of vacuum that can be sucked through it?
cos isn't it really creating a big vacuum leak for my carb?

i've capped it off at the carb and left it open on the engine side.
seems to run a fair bit smoother now.

I am thinking i probably should just find a new one...

I just wanna know what it does though. Restrict the flow so as not to creat a detrimental vacuum leak?
or should there be a flow restrictor on the other hose that go's to the air cleaner??

[Ron B.]

It is a Positive Crankcase Ventilation Valve or PCV Valve for short. This is the short explanation....

As a gasoline engine runs, high pressures build up in the cylinders and that pressure is contained within those cylinders. As an engine ages and the piston rings become worn some of the gasses created in the cylinders leak into the crankcase. This gas is referred to as "blow-by" as in "the gasses blow by the rings." If the crankcase is not ventilated in some way the seals would blow out. There must be a way to relieve those gasses and that is a function of the PCV system.

Prior to the use of the PCV system, engine builders used oil filler caps with vents and filter material. That allowed those "blow-by" gasses to escape to the atmosphere. Not a very nice smell. Then along came emission regulations.

Instead of being open to the atmosphere the crankcase is ventilated using a system, the PCV system. Fresh air from the air cleaner housing is supplied to the crankcase which contains the blow-by gas, oil mist and water vapor. The fresh air circulates in the crankcase and flushes airborne contaminants. The other half of the PCV system returns the crankcase gas and vapor to the intake manifold to be burned along with the fresh air and fuel supplied by the carburetor. A baffle or screen traps the oil mist before it drawn through the PCV Valve.

The PCV valve contains a variable restriction device which is controlled by engine vacuum, a spring and a ball or cone. The amount of vacuum that an engine produces varies. It is higher when decelerating and lower when accelerating. The vacuum works against the spring to vary the opening in the valve depending upon engine load.

A plugged or malfunctioning PCV system can damage an engine. Sludge can plug it. If you change your oil frequently sludge shouldn't be a problem. PCV valves have a long life due to the oil mist that gets by the baffle which keeps them adequately lubricated. It is generally recommended that the valve be replaced along with spark plugs or air cleaner elements.

I hope that helps.

Cheers!

Ron B. in Lake George, NY
'84 SR5 Wagon with 257k miles under partial restoration
'88 Fiero GT with 22k miles and driven daily
'65 Ford Econoline 5-window pickup under full restoration

[splatterdog]

A plugged or malfunctioning PCV system can damage an engine. Sludge can plug it.

If you're local climate can swing over a hundred degrees like mine, no PCV can get real ugly. Had one last winter horrible condensation and underhood emmissions.. My first thought was blown intake or head gasket. But it was caused by a plugged pcv port and was shooting rotten cappucino out the fresh air side and out the dipstick tube. Poor guy just bought it too and took the sellers word that it didn't leak oil. Augh! Still going though....

[toughtercel]

every oil change ,I normal check the PCV by putting clean hose on the outlet of PCV and suck it lightly to feel one way check valve working (bouns back and forth) because it so hard to pull PCV out of velve cover, it break most of the time, BUT looking at GM -F.I. Carb ,look like the port is plug ,I just realize it not just PCV to test but the whole line and connection from and to should fully check ,Interesting.I know Diesel doesn't have one , just curious newer model still no PCV or ?

[Petros]

sometimes you can clean a gummed up PVC valve with solvent, all it has to do it act as a one way valve so back fire pressure does not go in the crank case.

One thing I would add to Ron's excellent explanation, is that unlike most emission control devices, this can actually assist with the fuel economy, as well as reduce emissions, since the unburnt gasoline vapors are run back into the intake, rather than be vented to the atmosphere like they used to be. And when working properly does not affect max horsepower.

[gmeddy]

mmm very interesting.
thanks for the great explinations.
mine is deffinatly not gummed up.
i had the crank cover off the other day as i was doing the T.Belt, and i sucked and blew on it, it worked as it should, although i cant say if the spring in it is providing the nessesary resistance...?

so i guess i should just hook it back up then.

what about the other hose that is supposed to run to the air cleaner?
should it have any resistance or is free-flow ok?

Glen.
Tercel Tinkerer

PS. Splatterdog - what are them photos of??

[Petros]

so i guess i should just hook it back up then.

what about the other hose that is supposed to run to the air cleaner?
should it have any resistance or is free-flow ok?

Yes, hook it back up. I do not think it has a spring, the normal quick check is to shake it, if you hear it rattle, it is good.

The other hose is free-flow into crank case. The air is sucked from the air cleaner housing, through the crank case, up to the head and into the intake manifold through the PVC valve. If the blow-by gets too large (as the engine wears out), and the PVC system can not keep up with the normal operation, the intake tube acts like an overflow line, so the blow-by gasses will back up into the air cleaner.

[splatterdog]

4.3 tbi chevy truck, 88-93 vintage.

One pic is the plugged vacuum port on the throttle body for the pcv, the other is very wet blowby coming out the air cleaner hose all over the throttle body(the hose that allows fresh air from the air cleaner in to the crankcase). Both hoses should be unrestricted, they do sometimes collapse internally. The valve meters the flow.

If you ever get excessive oil in the air cleaner housing with a properly functioning pcv system it likely means your piston rings are giving up.