Have a question or want to start a discussion? Post it! No Registration Necessary. Now with pictures!
- Posted on
- How important is PCV valve?
May 17, 2005, 9:52 am
rate this thread
Yesterday I replaced the PCV valve in my 98 OBL, probably for the first time
in over 60K miles. The old one wasn't stuck but it did have a lot of crud
inside, and the outside of it was pretty rusty. In just the first mile of
driving I noticed that the motor seemed smoother, didn't hesitate and had
more low-end torque (upshift points were lower RPM than before).
Is this just a placebo effect or could that $3 part make such a difference?
Re: How important is PCV valve?
Don't know how much "blockage" your PCV really had, or if it seemed
clean but was sticky, but yes, this is one place a $3 part COULD make a
big difference. If it wasn't allowing enough flow thru the valve, excess
crankcase pressure had to go somewhere else. Could have been "bypassing"
thru other parts of the intake system, confusing some of the sensors and
consequently the ECU. Totally clogged PCV valves CAN cause blowby. Not
sure what the interval is for changing the valve on your car, but I
think most of mine have suggested 30k miles. And, of course, the guys
who sell the valves suggest 15k or so!
Glad to hear it helped!
Re: How important is PCV valve?
Hope this isn't too simplistic:
Your crankcase builds up pressure as the engine's running. This pressure
can come from "pumping" forces as the pistons go up and down just like a
compressor (except that the pressure's built below the piston in this
case as opposed to above it in combustion), and from blowby--compression
pressure leaking down past the rings.
This pressure has to be relieved or the engine can blow seals, and who
knows what else after that? Back in the dark ages when some of us were
kids, engines had simple "road draft tubes" that were nothing but a tube
generally coming from the valve cover area to vent all the pressure and
attendant gases to the atmosphere.
Problem is the gases in the crankcase--oil vapors, combustion
by-products, etc.--are pollutants. So sometime in the early 60's, mfrs
started (spurred by legislative inputs, of course) installing PCV
valves. The letters stand for "positive crankcase ventilation." The idea
is that these various gases would still be vented, but thru a one way
valve into the intake system instead of out to the atmosphere, thereby
allowing oil vapors and such to be burned. In general, PCVs today send
these vapors into the intake system after all the other sensors, so
they're added directly to the incoming fuel mixture. The "positive" in
the name is primarily a function of the way systems are designed with
some kind of cross flow or scavenging mechanism so there's some fresh
air flow throughout the engine, ensuring more of the "bad" vapors are
pulled out. Earlier systems (like in the '60s) were "open," in that the
air was often just pulled in thru a breather on the oil filler cap or a
similar source, then vented into the intake thru the PCV. Today's
systems are generally more "closed" in the sense they get their intake
air thru the air intake below the air filter and other sources.
If the PCV is plugged, pressure can build up in the crankcase. If it's
not badly plugged, vapors MAY be sucked into the system upstream of the
sensors, compromising their ability to function properly. In extreme
cases, a PCV system could become so plugged there might be a serious
blowby problem and blown seals, oil clogged air filters and other
problems. Much of the potential damage is a function of how the system's
designed in the first place: some are designed more "fail safe" than
Not a complete story of PCVs, nor 100% accurate, but I hope it helps.
- » High Mileage Preventive Maintenance Recommendations?
- — Previous thread in » Subaru Cars Forum