: What does a PCV valve do?

03-21-04, 11:39 PM
i was told i need to replace this, there cheap, so no big deal. but what does a pcv valve do? lol, i havn't replaced it ever since i got the car (1.5 years ago) so who knows how old it is. what happens when you don't replace it?


03-22-04, 12:21 AM
PCV stands for Positive Crankcase Ventilation. Basically, it takes the vapors from your crankcase, and recirculates them into your intake so that they may be burned off. This is a great thing, because as the engine gets hot, it burns off the impurities, such as water, that are in the oil. If the crankcase couldn't shed these impurities, they would stay in the oil and increase engine wear.

How do you know if one is good? Pull it off and shake it. If there is a rattling inside, it's most likely good. The rattling is actually the PCV check valve that reacts to engine vacuum. As vacuum increases (inversely to engine RPM, i.e. at idle, vacuum is highest) the valve opens and allows the crankcase vapors to enter the combustion process. Therefore, if you were to apply vacuum to the valve, it should open. Apply vacuum to the opposite side, and it should stay close. A bad valve leaks.

If you don't have a vacuum pump, and you're not squeemish, here's something else you can do to test a PCV valve. Wipe the oil off (unless you like the taste of 10W-30) and suck on the metal end. It should be tight, meaning, you should not be able to suck any air through. If you suck on the other end, it will always flow, and you'll end up with oil in your mouth. So leave it alone.

PCV valves are long lived components. But they are also cheap. So if in doubt, spend a couple bucks, and replace it.

03-22-04, 12:28 AM
i was told i need to replace this, there cheap, so no big deal. but what does a pcv valve do? lol, i havn't replaced it ever since i got the car (1.5 years ago) so who knows how old it is. what happens when you don't replace it?


The crankcase is purged of blowby gases and such by pulling fresh air thru the crankcase with engine vacuum . The fumes and vapor pulled from the crankcase are burned thru the engine to eliminate them. The PCV valve just controls the flow of the crankcase vapor/gases thru the vaccum line to the intake manifold. It has an orifice inside to control the flow. If you shake the valve you can hear it rattle. There is a loose piece inside and a spring so that the valve is pretty much open or unrestricted when there is no vacuum signal on it...this way when the engine vacuum is at zero (wide open throttle) the crank case pressure can vent thru the valve to the intake manifold via the vaccum hose. When there is engine vacuum present the loose piece inside the PCV valve shuttles to the closed position so that the flow due to engine vacuum is controled via the orifice in the shuttling part of the valve.

PCV valves are very very simple devices and rarely if ever need replacing in modern engines. The idea of replacing them regularily dates back to the late 60's when they were first introduced. I can't really imagine one failing or plugging up on a modern , well maintained engine. Replacing them is still recommended of course by those selling PCV valves...much like they still recommend 3000 mile oil changes...to sell more oil.

If you are remotely concerned for some reason that your PCV valve is not working....????....then just remove it from the cam cover and shake it to see if the shuttle valve inside is still moving freely. If it is then likely the PCV valve is fine. There is really no mystery inside of it to go wrong.

The PCV valve was actaully eliminated on the 2000 and later engines as there is simply a fixed orifice in the fitting in the cam cover replacing the PCV valve....that is how often they failed...they were just eliminated with a fixed orifice....LOL

If you look at your engine you will see the PCV valve on one cam cover connected to the intake vacuum with a vaccum hose. That pulls the fresh air thru the crankcase to eliminate the fumes and vapors inside the crankcase. The other cam cover has a fitting plumbed to fresh air in the air inlet duct ...that is the source of the fresh air for the crankcase...the vacuum pulls the air into the crankcase thru the fresh air hose. When the engine is at WOT the slight crankcase pressure can relieve thru the PCV valve (that is when the valve shuttles open so that it is unrestricted) and also thru the fresh air line.

An example of the importance of the PCV system is apparent on cold starts. On a cold start, especially in winter weather, several ounces of fuel and water get pushed past the rings and into the oil in the crank case on each start. As the oil warms up it starts to boil the water and fuel out of the oil . This is one reason you want the oil to get hot (above 212 F) so it can boil out all of the water and fuel to keep the oil from becoming contaminated heavily. The water and fuel vapor that is boiling off the oil is pulled out of the crankcase via the PCV system... so...the PCV system keeps the engine crankcase clean and burns the blowby gases by routing them thru the combustion chamber. If the PCV did not clean the crankcase of water and fuel and combustion gases blowing by the rings the oil would sludge up, deposits would form, etc.... PCV is a good thing.

03-22-04, 12:41 AM
Yes. What he said. As I was writing, I couldn't remember if the PCV valve is activated by vacuum, or lack thereof....:hmm: And it's too damn cold outside, to check.:hide:

03-22-04, 01:54 AM
I can tell you exactly what can happen if you DON'T ever change it, or it freezes. I've never had a problem with my GM vehicles and the pcv valve, but with my 1991 Merc 5.0, the pcv valve and/or the little round wire mesh pre-pcv valve filter frooze up and cost me 1,200 dollars in oil seals! I suspected that it was the pre filter, as it looked like a solid chunk of ice in our cold climate, after years of never being changed. (even the Ford mechanics knew nothing of the pre-filter directly underneath the pcv valve) That is the cost of a new computer in Canada, and I still only have 166 mhz! :annoyed: The engine has to be vented and breathe, (generally speaking) or the pressure will tear through your oil seals, as it has to escape somewhere. Do yourself a favor and change it for cheap insurance every once in a while, it can't hurt. I changed the pcv, and the mesh filter immediately after that event. If you don't hear rattling when you shake it, you can clean it out with gasoline, as they can get clogged with oil, etc.

According to my owners manual, (and Cadillac) it should be changed every 30,000 miles (50,000 kms) at the same time change the air filter as well. The pcv valves are very cheap, less than 5 dollars, no excuses then, right?

03-22-04, 12:42 PM
Certainly a lot of harm can occur to the engine if the PCV system does not function correctly or gets plugged or something. Icing of the PCV system is a design/validation characteristic that receives a lot of attention...at least by powertrain engineers that understand it and what can happen if it doesn't work right.

The Toyota engines that had the high failure rate with sludge formation basically had a problem with the PCV system not sufficiently cleaning the crankcase of water vapor when the vehicle was subjected to numerous cold starts with little warmup.

If the system is a "closed" PCV system (no fresh air makeup) then pressure can build up if the PCV valve freezes or is blocked and the crank case pressure goes too high as descibed in the example above.

There are several different types of PCV systems so do not characterize the failure modes of one type of being "typical" PCV failures.

The worst case scenario with theoretical PCV systems involves substantial water buildup in the oil from condensation over time (blowby from cold starts) and eventually the water puddles in the bottom of the pan...where it can freeze in cold weather and the chunk of ice can block the oil pickup and starve the engine for oil.

03-23-04, 12:06 AM
Absolutely Bob! Plus, I think the terrible, IMO, design of the old Ford 5.0 pcv was stupid to say the least. I won't be buying a Ford or Mercury for a long time, if ever, because of that costly incident. I know that with the newer 4.6 in the Crowns and Marquis, the pcv valve looks similar to the typical "L"shaped devive GM uses. But it's pretty bad when the factory trained Ford and Mercury mechanics didn't change the pcv, or pre-filter for me when they repaired the car. It could have happened again, despite them telling me that was the problem! I think it's safe to say, some things should be looked into, or repaired if possible, by the owner themselves.

06-02-13, 05:49 PM
if you have a 99 cadillac deville the pcv valve is on the passenger side in the back next to the cover with the firing order stamped on it. pull it straight up and then be careful when you pull the pcv valve out of the elbow use a screwdriver to gently pry on the elbow. because,its old and dry and will break. they're just a few dollars if you do happen to break it.

06-02-13, 06:35 PM
That's some DEEP searching - a 9 year old thread !!! ............ but it's still a legitimate question because a LOT of car owners have no idea what PCV does - or that the "rattle valve" is a maintenance item that can cost oil consumption and poor fuel mileage if left unattended.

One of the respondents in this thread was a GM Powertrain Engineer in the Northstar Project.

BTW - Welcome Aboard !!! - :yup: