PCV Valve

Original Author: Phil Barnett (prb) - 2013

Back to Maintenance

What is a PCV valve?

A PCV valve is an emissions control. It's a simple one, but it's very effective in its job. Many years ago, the interior of every engine was vented to the outside atmosphere through what was called a down draft tube. If you had leaky rings or valve guides and the pressure of combustion caused leakage into the crankcase, the pressure was relieved out through the down draft tube. Of course, this would mean venting unburned hydrocarbon gasses directly into the atmosphere. Usually, these fume leaks that pressurize the crankcase are worse under hard acceleration. Well, that makes sense, doesn't it?

Anyway, laws were passed to eliminate unburned hydrocarbons from being vented directly into the atmosphere and the down draft tube was one of the first things to go. It was replaced by the simple PCV valve. PCV is an acronym for Positive Crankcase Ventilation. It's job is to scavenge all those unburned hydrocarbon fumes and transport them to a place they will be burned up/consumed.

Essentially, the PCV valve is a big metered leak of air into the intake system drawing all of it's input from the crankcase. Of course, a big air leak can't happen all the time or the vehicle wouldn't idle worth a damn. The PCV valve receives it's signal on when to leak from the number of inches of vacuum in the intake manifold. It turns out to be a good indicator of when there will be a lot of fumes to scavenge. If the throttle is shut, the vacuum is high and there isn't much fumes to scavenge. When the throttle is open, the vacuum is low and there is a lot of fumes to scavenge. The PCV valve sucks shut at high vacuum which stops the leak when the throttle is closed and the least horsepower is being generated. The PCV valve opens when you step on the throttle and the vacuum in the intake manifold drops.

The normal operation of the PCV valve is to suck closed at around 13" of vacuum. That mostly means it will be sealed when idling and just starting to open on very light acceleration.

Testing your PCV Valve

The typical failure of a PCV valve is that it sucks shut, but doesn't seal, leaking at an idle and possibly causing a high idle. Usually you have to replace them once that happens. You can try cleaning them but they cost too little to spend much time on.

Another failure mode is that it sucks shut too early because the spring that determines how many inches of vacuum causes it to actuate has weakened. For sure you have to replace them when that happens.

The leak test is made a little more difficult by our cars having a Idle Air Control Valve, so bypass the engine control computer by jumping TEN to GND.

Next, pull the PCV valve from the valve cover.

With the engine idling and listening to the idle speed, cover the exposed end of the PCV with your thumb. The engine should not change idle speed at all. (because at idle, the PCV should already be pulled shut due to high vacuum). When you let your thumb off, the engine will surge while the valve is being pulled shut and then it will go back to normal speed. That surge on release is normal and does not indicate a failure.

If the PCV is leaking, the engine will change speed for a moment when you close the valve with your thumb, because your thumb supplies the seal that the PCV should have been supplying.

It's normal for any engine to surge idle when you release your thumb. That's not part of the test.

Another feature of your PCV valve that isn't often seen in a typical engine is to close if the intake manifold becomes pressurized. This can happen if there is an explosion in the intake (that would be bad) or if you happen to have a turbo or super charger on your car. In any case, you don't want that pressure to pressurize the inside of your crankcase so a typical PCV valve will seal when there is any pressure from the intake manifold.

It takes very little pressure to pressurize a PCV valve shut in the opposite direction. So you might be able to accomplish that with your lungs.

A PCV valve should rattle if you hold it in your hand and shake it. That's the valve mechanism moving around at room pressure. It doesn't have anything to do at room pressure so it sits loose in the valve.

There is no blow through it test that makes any sense. Your lungs cannot reach the vacuum or pressure it takes to close the valve in it's normal operation and unless you are turbo or super charged, the backwards valve will never come into play. (and that's the one you might be able to test with your lungs)

Substitution is the ultimate test of any part. It's a great diagnostic test that only gets expensive if you are substituting expensive parts. For inexpensive parts, it's worth replacing just to eliminate the part. PCV valves are inexpensive to substitute.

There's four things a PCV has to do.

  1. Pull shut and seal above around 13 inches of vacuum. (idling or cruising, high vacuum)
  2. Open when vacuum falls below about 10 inches of vacuum (accelerating or room pressure)
  3. Meter the flow when it's open. (how big is the metering hole in the base?)
  4. Seal if the intake becomes pressurized (turbo or super charger)
It generally won't do 1, 3 or 4 at room pressure or by blowing through it. So you can rattle it and that's OK. And you can blow slowly through it both ways and that's OK because you don't reach the volume of air that it takes to activate it.

If it doesn't rattle at room pressure, it's bad.

A bad PCV valve can also make the throttle feel imprecise. It's designed flow pattern falls right between cruising and accelerating so if it's wrong, that line becomes blurred.

There are part numbers for aftermarket PCV valves in the Tune Up Parts List, but I much prefer the factory PCV valve. It makes my throttle feel the best of all of them I tested.