March 23rd 2007 

Even on the most expensive and sophisticated vehicles, there are a couple of engine control items that date back to the stone age.

One of these is the PCV (positive crankcase ventilation) valve and the other is the thermostat, the latter we will deal with next week.

Neither of these items has benefited at all from electronic control techniques and both of them continue to rely on mechanical
operation for their ability to perform.

Which also means that either one of them can fail without much notification for the technically challenged car owner.

What, then, does a PCV valve do and why should I pay to have it changed?

Well, as an engine runs, the crankcase collects combustion chamber gasses which leak past the piston rings.
These combustion gasses are sometimes referred to as "blow by" because the combustion pressure "blows" them
"by" the pistons. These gasses are harmful to an engine. They also contain a significant amount of water vapour and the
internal pressure created can cause engine oil seals to leak.

If allowed to remain in the crankcase, or become too concentrated, the harmful compounds begin to condense out of the air
within the crankcase and form corrosive acids and sludge on the engines' interior surfaces.

This can clog small inner passages, causing overheating, poor lubrication, and high emissions levels.
To keep the crankcase air as clean as possible, some sort of ventilation system must be used.

Before the 1960s, automobile gasoline engines ventilated directly to the atmosphere through a simple vent tube. 

Frequently this consisted of a pipe that extended out from the crankcase down to the bottom of the engine.
The bottom of the pipe was open to the atmosphere, and was placed such that when the car was in motion a negative pressure
might be obtained, helping to extract combustion gasses as they collected in the crankcase.

The system was not positive, as gasses could travel both ways. Older readers will remember seeing engines in bad condition
blowing clouds of blue smoke out from underneath the car and if you were following the offending car, the smell was something
one wanted to get away from as fast as possible, usually with a desperate passing manoeuvre.

In the mid 50s it was discovered that the vent tube was a major source of the unburned hydrocarbons coming from the automobile.

By legislation, therefore, the PCV valve became a required feature on all cars

In the event of a backfire, the PCV valve closes to prevent reversal of the exhausted air back into the crankcase again,
preventing explosive internal engine fires.

It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will be insufficient.

A plugged or malfunctioning PCV system will eventually damage an engine. PCV problems are primarily due to neglect or
poor maintenance, typically engine oil change intervals that are inadequate for the engines driving conditions, although the
rising use of synthetic engine lubricants makes this a far less critical factor than it used to be.

A poorly maintained PCV system will eventually become contaminated with sludge, causing serious problems.

If the engine's lubricating oil is changed with adequate frequency, the PCV system will remain clear practically for the life of the
engine. However, it will inevitably fail over time.

Typical maintenance schedules for gasoline engines include PCV valve replacement, usually when the spark plugs are replaced.

The long life of the valve despite the harsh operating environment is due to the small number of oil droplets suspended in the
air that flows through the valve which keep it lubricated.

The PCV valve is only one part of the PCV system, which is essentially a variable and calibrated air leak, by which the engine
returns its crankcase combustion gasses for further combustion.

Instead of the gasses being vented to the atmosphere, gasses are fed back into the intake manifold, to re-enter the combustion
chamber as part of a fresh charge of air and fuel. The PCV system diverts a small percentage of fresh air via the breather to the
crankcase before allowing it to be drawn back in to the intake tract again. It is an "open system" in that fresh exterior air is
continuously used to flush contaminants from the crankcase and into the combustion chamber.

The system uses the lower pressure of the intake manifold to draw air in, pulling air from the breather through the crankcase
through the PCV valve, and into the intake manifold.

The PCV system consists of a breather tube , and a PCV valve. The breather tube connects the crankcase to a clean source
of fresh air, such as the air cleaner body. Usually, clean air from the air cleaner flows in to this tube and in to the engine after
passing through a screen, baffle, or other simple system to arrest a flame front, to prevent a potentially explosive atmosphere
within the engine crank case from being ignited from a back-fire in to the intake manifold.

The baffle, filter, or screen also traps oil mist, and keeps it inside the engine.

Once inside the engine, the air circulates around the interior of the engine, picking up and clearing away combustion by-product
gasses, including a large amount of water vapour, then exits through a simple baffle, screen or mesh to trap oil droplets before
being drawn out through the PCV valve, and into the intake manifold.

The PCV is simple, but actually performs a complicated control function.

An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring,
exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is
drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension.

At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV
valve provides the largest amount of restriction. As engine load increases, vacuum on the valve decreases and blow by increases.
Sensing a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow.

At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gasses escape
via the breather tube where they are then drawn in to the engine's intake manifold anyway

How can a PCV valve be tested? Not that easily.

As click and Clack the Tappet brothers are wont to say "The best test is to throw it on the floor and stamp on it!"

For the $25 involved in replacement and considering how important this little device remains, the next time your garage
tells you that  you need a new one, don't hesitate, replace it.