PCV Valve & Breather Hoses

What is the PCV Valve and breather hose and where is it located? 

The Positive Crankcase Ventilation (PCV) valve is an emissions part that clears gasses from the crankcase.  The crankcase houses the crankshaft and the oil that keeps the crankshaft and pistons lubricated.  It is separated from the combustion chambers by gaskets.  Sometimes, though, gasses can sneak past the pistons and the gaskets into the crankcase.  These blow-by gasses can froth up and contaminate the oil, making it less effective at lubrication, and the added pressure can damage the crankshaft and engine seals. 

There has to be some way to vent the gasses out of the crankcase.  In older cars, there was simply a tube that vented gasses outside the car into the open air.  This was tough on the environment and a waste of fuel.  So, the PCV valve was invented to pass blow-by gasses from the crankcase to the intake manifold, where uncombusted fuel in the blow-by gas could be burned. 

The PCV valve can be attached to the valve cover, with a breather hose running to the intake manifold, or attached to the intake manifold, with a breather hose running to it from the valve cover.  As gasses are vented out of the crankcase, fresh air has to be vented in.  So there is also a breather hose that draws air from the intake hose into the crankcase.  

Pushing blow-by gasses to the intake isn’t always good for the engine, so the PCV Valve has a plunger that adjusts the gas flow in response to pressure in the crankcase and vacuum in the intake.  When the engine is at idle, intake vacuum is high and crankcase pressure is low.  That pulls the plunger all the way forward, closing the valve.  If the vacuum was allowed to simply pull gas out of the crankcase at this point, it would make the fuel/air mixture too lean.  It might also draw oil out of the crankcase into the engine.  During acceleration, high vacuum in the intake also pulls the plungers forward and restricts airflow.  When the engine is under a light load, like when you’re cruising on the highway, there is less vacuum, which pulls the plunger only partway forward.  That allows gasses to flow around it.  In these conditions, blow-by gasses can be combusted, without major effects on power or fuel efficiency.  When the engine is off, a spring pulls the plunger all the way back, closing the valve.  If there is back pressure in the intake manifold, from a misfire for example, the pressure will push the valve all the way back and closed. 

How do I know if my ­­­­PCV Valve or breather hose needs to be replaced?            

The blow-by gasses and any oil that get drawn through the PCV valve can leave residue inside the valve.  Eventually, the valve can get clogged.  It might get stuck in an always on or an always off position.  If the valve is always on, it will continuously pass blow-by gasses to the intake.  That will make the engine idle roughly and might make it hesitate in acceleration.  If the valve is stuck, closed pressure will build up in the crankcase and might damage seals and gaskets.  If you have engine oil leaks in many locations, a blocked PCV valve might have blown your seals.  Whether the valve sticks open or closed, it can cause you to fail a state emissions inspection. 

There are a couple simple tests you can do to check your PCV valve.  The first and easiest is to pull it off and inspect it, visually, for oil and residue.  If the PCV valve looks clogged up, you should replace it.  You can check the PCV valve and listen for a rattle.  This is one case where a rattling sound is a good sign.  No rattling means your valve is definitely stuck.  Next, clean the valve up a little bit and try to blow and suck air through it.   Air should flow in one, but not both directions.  If air flows both ways, the plunger is too loose.  If air won’t flow either way, the valve is clogged.

As for the hose, like any other hose under your hood, breather hoses can wear out over time in a number of ways.  With heat and age, they can become brittle and cracked.  Oil dripping on the outside can also soften up the hose.  In either case, the hose may begin to leak.  On the other hand, blow-by gasses and any oil that gets pulled through the hoses can leave behind residue that clogs up the hose.  These problems can make the PCV valve work less than optimally, causing problems like rough idling and hesitation during acceleration if there is too much gas flowing through the PCV valve; or high pressure in the crankcase, blown seals, and oil leaks if the PCV valve allows too little gas to flow.  If you are experiencing any of these problems, you might want to visually inspect your breather hoses.  If the hoses seem worn, replace them.

Can I replace the PCV Valve or breather hose myself?

The whole business of venting gasses to one place to another, and only under certain circumstances might sound complicated, and in some ways it is.  So, you might expect that replacing a PCV valve or breather hose would be complicated, but, actually, it’s not.  In many cases, accessing the valve or ends of the hoses is the hardest part.  If the valve is easy to access, you can probably replace it by hand.  Otherwise, needle nose pliers can be a big help for both.  You’ll just have to pull the breather hose off the valve.  This is a good time to inspect your breather hose for wear or damage.  Then you can pull out the valve itself.  In some cases, it pulls out; in others, you’ll have to unscrew it.  Simply put the new valve in the way the old the one came off and attach the hose.  If you’re having any trouble getting the valve to seat correctly, you can use motor oil to lubricate it.

As a side note, you should always buy breather hoses designed specifically for your vehicle.  Some people will try to tell you that you can substitute fuel hoses for breather hoses.  That won’t work.  Breather hoses are designed to resist being pulled in by vacuum.  Fuel hoses are built to resist internal pressure.  A fuel hose used in place of a breather hose will collapse, leaving you with basically the same problem that you started with.