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Replacing the PCV (Positive Crankcase Ventilation) valve, and why you need to
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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 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.

There were two problems with this road draft tube system:

• When emissions controls first came in, it was recognized that the road draft tube contributed to smog by venting unburned hydrocarbons (raw gas) to the atmosphere
• The air inside the crankcase was essentially static except for what was pulled out by suction near the road. If the car wasn't moving, crankcase air flow suffered.
  

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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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.

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

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...

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.

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.

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.

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

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.

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.

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...

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!

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

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.

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...

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.

I reassembled everything as well as I could to see how it worked: back to top 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.