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Replacing the PCV (Positive Crankcase Ventilation) valve, and why you need to
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What happens when it gets clogged and how to keep it fron getting that way
What happens when you remove it entirely in a modern car?
Replacing the PCV valve
How it works

What happens when it gets clogged and how to keep it from getting that way
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Well, to get to the heart of the matter, you need to CHANGE YOUR OIL at timely intervals! That, and that alone, will prevent almost all PCV valve problems. PCV valves plug up with sludge. Sludge is old oil. If you need to change your PCV valve, chances are somebody's not been changing his oil often enough for driving conditions. And chances are good that you'll need to some serious cleaning of lots of other things on the engine, like the throttle body and Idle Air Control Valve.

The system consists of a small air intake breather hose mounted in the intake tract before the throttle body, and the PCV valve, which is typically in the intake manifold or the valve cover. Air flow is from the breather before the throttle body, through the crankcase, up the PCV valve and into the intake manifold after the throttle body. It's basically a controlled air leak, with the PCV valve acting as a one-way check valve that has adjustable flow characteristics.
Some diagrams to help:

If the PCV valve gets plugged, here's what happens, roughly in order:
  1. Excess crankcase pressure begins to back up the engine's breather tube, into the intake air tube upstream of the throttle body
  2. Engine management system senses absence of air from plugged PCV valve, and adjusts fuel mixture accordingly. Engine makes slightly less power.
  3. Oil begins to be pumped into the air intake tube through the breather hose, upstream of the throttle plate.
  4. Oil runs down the intake tube into the throttle body.
  5. Oil sludges up the throttle body and throttle plate. Gas pedal starts to get sticky. Airflow reduced. Engine management adjusts mixture again. More power loss, possible poor driveability.
  6. Idle Air Control (IAC or EACV) valve gets contaminated and begins to stick.
  7. IAC cannot adjust idle air mixture to compensate for plugged PCV valve. Idle drops, may become erratic as throttle plate cannot close completely.
  8. Oil drips into intake manifold runners. Blue smoke seen out of tailpipe.
  9. Breather tube begins to get constricted from oil contamination and airflow is reduced. Crankcase pressure mounts dramatically.
  10. Oil pushed past valve guide oil seals. More blue smoke.
  11. Dripping on driveway as oil is pushed past seals.
The above is the scenario for a modern car with a feedback engine management system, in other words, one with an oxygen sensor. Older cars without engine management systems were unable to compensate for plugged PCV valves, and the power loss, fuel mileage loss, rough running issues were FAR worse.  A PCV valve that was stuck open could even result in a refusal to idle at all.

The function of a PCV valve is to control the flow of crankcase gases. Start your engine, then remove the oil filler cap (watch your clothes; you might get some oil splatters). Place your hand just above the opening. Feel the strong jet of air? That's "blowby", combustion gases that squirt past the piston rings into the crankcase below the pistons. Even new engines in good condition generate lots of blowby (but it's still a small
fraction of what passes through the throttle body).That jet of air has to be vented somewhere, otherwise it will quickly build up pressure inside the engine, and cause those aforementioned very bad things to happen. If the PCV valve gets plugged, blowby pressure has no proper place to go.

The primary reason for plugged PCV valves is neglected oil changes, which causes sludge. Secondarily, it also happens to engines that are primarily used for short trips, where moisture, fuel, and acids build up in the oil, emulsifying it into sludge, which is related to the neglect thing mentioned earlier. 3 months between changes might be too long depending on how you drive.
Also, an engine with worn compression rings will have massive amounts of blowby, which will overwhelm the PCV valve's ability to pass air and cause the excess vapors (and oil) to be pumped into the intake before the throttle plate.

Since the air inside the crankcase also contains contaminants like unburned fuel, water, acids and other things that your engine and its oil do not like, venting the excess pressure also helps to vent those contaminants out of the engine before they settle into the oil in the first place, so your oil stays a bit cleaner longer.

In the old days, you had something called a "road draft tube", which simply allowed the pressure (and the gases) to leak out a hollow tube that extended downwards from the crankcase to a spot near the bottom of the engine, where it dissipated into the surrounding air.

This is a Hudson straight-eight. It has not one, but TWO road draft tubes, indicated by the arrows. The tubes come out of the valve chest just below the manifold and, descend to the bottom of the oil pan.
Hudson Straight-Eight showing draft tubes

There were two problems with this road draft tube system:

The answer to the emissions problem, first mandated by the State of California around 1960, was Positive Crankcase Ventilation, where the engine was supposed to swallow its own blowby. This meant a closed-loop system: The road draft tube had to go. In place of the road draft tube was the famous PCV valve.

Some PCV valves are very simple to replace. They sit on top of the cam cover, right in plain sight. You grab them, haul until they pop loose, then replace them.

Replacing the PCV valve
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Toyota PCV valve location
This Toyota Tercel's PCV valve is a case in point. Like late-model Civics and Integras, it's in plain sight on top of the cam cover, and is easily yanked out to replace.

By the way, even a V6 has only one PCV valve. It will be located in one of the two cam covers.

Toyota PCV valve closeup
A closeup of the above photo.  The PCV valve is the white thing.

Well-hidden PCV
However, on some cars, such as my '91 Integra and certain Civics and Accords, the PCV valve is in a rather odd place. See the red arrow in the picture to the left? If you click on the picture, and you look very carefully to the immediate right of the point of the arrow, you'll see the vague dark shadow of the PCV valve, located in a web of aluminum between the #3 and #4 intake manifold runners, under the fuel rail. The upper hose goes from that to the intake plenum. The lower hose? Well, keep reading...

Moving the clutch cable
The first thing we need to do is improve access, so the clutch cable needs to be moved. Your car will be different from mine, so use your judgement as to what you need to do here.

I undo two of the cam cover nuts (being careful not to lose those acorn nuts), and move the clutch cable back, hooking it over a handy vacuum valve.

Wiggling other end of PCV hose to loosen it
Before I pull the valve out of its hole, I want to loosen the other end of the upper hose. I do this by gripping it gently with the same pair of angled needle-nose pliers I'll use to remove the valve, and rotating it either way to break the seal caused by time and corrosion.

Don't squeeze too hard or you'll distort the metal stub.

Using angled pliers to pop old PCV loose
A set of angled needle-nose pliers is essential to be able to grab the old PCV valve and pull it out of its hole in the lower hose.

What also helps is a warm (preferably HOT) day, and a hot engine, so everything is loose and flexible. I had originally attempted to remove the PCV valve at one other occasion, that being in January. The valve absolutely would NOT budge.

Finally out
Now that the valve is popped out, I can pull the other end of its upper hose off the stub on the intake plenum.

Open PCV hole in lower hose
And there's the hole that's left behind. That's the upper part of the lower hose. I carefully wiped off the sand with the fingertip of my baby finger, making sure nothing dropped inside the lower hose.

New parts
Here are the new parts I intended to install. The PCV valve is at the top of the pic, and the lower hose at the bottom. My  rationale for replacing the lower hose is that I figure that after 14 years, that grommet has to be slightly shrunken, leading to air leakage around the PCV valve.

I notice the new PCV has the same part number stamped into it as the 14 year-old original, so Honda evidently never updated this part, except to make the plunger blue instead of green.

Bottom hose
Now, how do I get this puppy in there? I removed the intake hose so the throttle body was fully exposed, but no matter how I wriggled my arm under the intake manifold, all I could do (if my arm was juuust so...) was touch the lower hose with my middle fingertip.

Looks like I'd need to remove at least the thermostat housing and the heater hoses before being even close to being able to pull the lower hose off the intake manifold and the oil separator box. OK, forget that. I'll take my chances leaving the old lower hose in...

Pushing new PCV in place
Installing the new PCV was simplicity itself. I installed the PCV valve to the upper hose BEFORE pushing the PCV valve in place.

WARNING: NEVER use silicone to grease the new PCV valve before insertion! These parts are designed to be in the presence of motor oil, so smear a bit of your choice of motor oil on the bottom of the PCV valve before pushing it into its grommet. Silicone and oxygen sensors do NOT get along!

New PCV installed
And there we are. New PCV valve in place. It's pretty easy, actually. Warm weather combined with needle-nose pliers did the trick.

Torquing cam cover nuts
As a final step, I move the clutch cable back, then torque the cam cover nuts back down to 7 ft-lbs.

Don't overtighten, or you'll damage something! A decent beam-type torque wrench is all of $20 at your local parts emporium, and is more than suitable for this sort of job.

New and old valves
I decided to remove the PCV valve only because I thought it had to be at least slightly clogged after 14 years and 248,000 miles. There was no other reason for me doing this.

Well, at first blush, the old one looked pretty grungy compared to the new one. However, when I wiped them both off and blew through them there was NO DIFFERENCE AT ALL. They both exhibited identical resistance to breath pressure no matter how hard or soft I blew.

Time to see what's inside...

Sawed apart - 1
This PCV valve was glued together and was not disassemble-able as far as I could see, and the plastic was far too hard to cut with a knife. Out comes the hacksaw.

Now it becomes clear why both valves allowed the the same air passage when I blew into them: The old one looked BRAND NEW on the inside! There was not even a trace of oil or sludge inside! ALL the deposits were on the outside faces.
Must be all those 3K mile or 1½ month oil change intervals...

See that little silvery curl to the left? That was the spring that held the green plunger down at low pressure differential.

Sawed apart - 2 I reassembled everything as well as I could to see how it worked:
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The valve is normally closed by virtue of the curly spring. The green plunger is like an upside-down roofing nail. Its head sits on the bottom (crankcase side) of the valve body against spring pressure, preventing gas passage.

There are actually two "closed" positions for the PCV valve. One "closed" position keeps intake air from entering the crankcase, and the other keeps most blowby gases from entering the intake. In between is the "open" position, where gas is allowed to pass most easily to the intake from the crankcase. The black mark you see is the point of highest plunger lift, and highest gas velocity. Soot collects there like it does around your throttle plate.

I had originally thought there was no taper on the plunger on Honda PCV valves, but there is. On the shaft of the plunger from the lowest to highest points that I can measure accurately on my example, there is a .014" taper. Also, my example's high mileage (248,000 miles) appears to have worn a small step into the shaft, which stops right at the point of highest vacuum (at the bottom of the black mark). This step is .006". Both taper and step are hard to see unless you REALLY stare at it.

Toyota PCV valves have an angled slot or a large and obvious step in the shaft of the plunger. These accomplish the same thing as the taper on Honda valves.

At high positive pressure differential (crankcase is higher than intake), flow becomes most restricted as the plunger rises and its shaft wedges its bore mostly closed.

At negative pressure (intake is higher than crankcase) the plunger drops and the head reaches its seat, sealing the valve shut, preventing intake air from entering the crankcase. That's where the Positive in Positive Crankcase Ventilation comes from: Air either moves from crankcase to intake, or it stops entirely.

In between these two extremes is the plunger position where most gas flow occurs from crankcase to intake. At some point, gas passage area around the plunger head equals gas passage area at the plunger shaft's bore, and gas flow is maximized. This maximum flow can occur at just about any throttle opening as long as pressure differential is at just the right level.

At full throttle, there is initially little pressure differential between crankcase and intake since the throttle is wide open, but blowby will also be at its highest level, so the valve will be pushed open once that full-throttle blowby hits it.

The clicking noise you hear when you shake a good PCV valve is due to the fact that the spring that is supposed to push the plunger head against the seat is actually slightly short. It stops just short of the seat, so at very small pressure differential, the plunger's head can rise off its seat impeded by no spring resistance at all. This probably helps to ensure operation at extremely low pressure differentials.

I removed and cleaned the EACV, and cleaned the throttle body as well, but that's another page (not written up yet).

What happens when you remove the PCV valve entirely in a modern car?
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There are many, many Web sites that report that a stuck-open PCV valve "can" result in rough idling and stalling. This seems to be standard  PCV system boilerplate that has not been updated since 1980. Just to see what happened, I tried uplugging the PCV valve from the intake manifold on two cars, a 1991 Acura Integra, and a 1999 Toyota Tercel. The Tercel, of course is OBD-II, which means its engine management system is pretty sophisticated.

The 1991 Integra:
1)  I tried plugging the PCV hose shut. The idle dropped momentarily, then climbed smoothly back up to 750rpm and remained there.

When I let go of the hose, the idle surged briefly, then settled back to 750rpm. As expected, the engine management system was adjusting air flow to achieve correct idle speed.

2) with the car fully warm, I unplugged the PCV valve hose at the pipe where it enters the intake manifold, so the PCV intake was completely open as a massive air leak into the intake downstream from throttle plate.

The idle dropped momentarily, then began smoothly and regularly surging between 1,000rpm and 2,500 rpm, with about one second between cycles . As expected, the engine management system, crude as it is, was attempting to adjust the mixture to be correct. The air leak being too great, however, it was unsuccessful in doing so.

(Interestingly, air flow from the PCV valve itself stopped dead, suggesting that ambient air pressure was keeping the valve shut. In other words, crankcase pressure was lower than ambient, probably because excess pressure was being drawn into the intake tube upstream of the throttle body.)

I then plugged the PCV intake pipe with my thumb, mimicking a plugged PCV valve. The idle returned to a smooth 750rpm and stayed there.

I discovered that I could uncover up to a quarter of the PCV intake pipe before the idle began surging again, the severity of the surging directly related to the amount of exposed intake pipe. A quarter of the intake pipe appears to be roughly what would be admitted by the PCV valve itself at full-flow.

It appears that given a massive air leak, the engine's computer will continue to attempt to adjust the mixture by adding fuel until the RPMs rise to the closed-throttle fuel-cutoff point, at which point fuel is turned off, the revs fall enough for the ECM to turn the gas back on again, and then the cycle repeats until the leak is plugged or the car runs out of gas.

The 1999 Tercel:
1): The hose-plug test was identical to the Integra, except that the idle never dropped. It appears that the Tercel's engine management system is faster than the Integra's, and responded too quickly for the idle to drop. Idle remained the same.

2) When I pulled the hose off the intake, creating the same massive air leak I had subjected the Integra to, again the idle did not drop, but immediately began a surge cycle. The Tercel has no tach, but it did not seem to rev nearly as high as the Integra, and its cycle time was about a half-second instead of the Integra's full second. The idle settled down immediately once the pipe was plugged with my thumb.

I discovered I could uncover fully half the exposed intake pipe before cycling began. Slowly uncovering the pipe resulted in a steadily INCREASING idle until surging began.
Half of the intake pipe appears to be roughly what would be admitted by the PCV valve itself at full-flow.

To repeat from above: It appears that given a massive air leak, the engine's computer will continue to attempt to adjust the mixture by adding fuel until the RPMs rise to the closed-throttle fuel-cutoff point, at which point fuel is turned off, the revs fall enough for the ECM to turn the gas back on again, and then the cycle repeats until the leak is plugged or the car runs out of gas.

Conclusion: Cars with engine management systems do not suffer noticeable problems from a plugged or stuck-open PCV valve unless an extremely unusual situation is present.

Some history:
I once had a 1975 Toyota Corolla with a carburetor and no feedback system. This car would not idle at all with the PCV valve removed. Unlike the Integra's and Tercel's  feedback systems, the carburetor was unable to compensate for the excess air. I discovered this quite by accident when an aftermarket PCV valve came apart on me.

I had removed the valve cover (the PCV valve was perched atop it) to adjust the valves, and placed the cover upside down on the driveway. Unbeknownst to me, a little ball fell out of the PCV valve and rolled away. On reassembly, the car started fine, but would only run if the throttle was pressed. I pulled my hair out for a good hour, checking and rechecking everything, and I could find nothing out of place or otherwise wrong. I don't remember what made me pull the PCV valve out to check it, but I did, and to my astonishment, I could see right through it! I had a giant air leak! No wonder the car would not idle: There was insufficient vacuum under the throttle plate to help pull fuel out of the idle jets.

I eventually found the little ball at the edge of the driveway, cleaned it off, put it back, and everything was back to normal. I later discovered that OEM Toyota PCV valves would not come apart on you if you tippped them upside down. So guess what I did next?