The Volvo/ FORD ST pcv system  is made out of two parts like most cars.

There is high vacuum /low throttle side, and then wide open side ,Low vacuum high positive pressure side and its a complete closed system.

Now lets look at the high vacuum side

During this state oil vapor inside the engine is sucked into the intake manifold through the oil filter housing which is open to the bottom of the cylinder block and connected to the cam cover.

Inside the OFH you will find a diaphragm which regulates the flow as the vacume pull is rather strong, strong enough to suck in the oil seals on the cams and the crank , causing them to leak.

This particular diaphragm also act as a shutof valve when the car is in boost, but helped by the little reed vavles situated inside the manifold runner gasket.

Now the WOT/High positive pressure side

Under wide open throttle the vacume in the intake manifold turns to positive pressure. Now the little reed valves gets blown againt their seats to close the system from the pressureto make sure no pressure gets past them and pushes on the diaphragm and all is well.

In turn there is still alot of vapor pressure and positive pressure inside the block that must go  somewhere, this is where the secondary system kicks in

Volvo utilizes an anti pollution regulating valve at the turbo, now this is connected at the throught of the turbo which under boost will suck on this valve which is connected via the lower manifold to the cylinder block, oil vapour gets pulled into the high pressure air stream pushed through the intercooler and piping and the burnt inside the combustion chamber

And all is well! 

BUT 

Thats where the problem stems off.

So in the past we focussed on the Boost / high pressure side  

Some boost leaked past the reed valves, causing pressure on the Diaphragm located in the OFH, causing it to rupture.

End result was usually a high pitch scream at idle.

Our fix at the time was a swing action one way valve with an enclosed catch can.

It worked, saving numerous diaphragms in its stride  

BUT

then we found more and more Tuned cars with a smoking habit!

Now where do we start......

So we started looking at the low pressure / vacuum side 

For some reason oil mist must be suck  or blown into the combustion chamber.

We tried venting the pressure side . Which didnt help as the car would suck in un measured air and would run lean under vacuum............................................

On the ST MAF pin 1 is not used.
- On the RS MAF pin 3 is not used.

- Pin 2 on the ST goes to Pin 1 on the RS MAF GN/BU wire
- Pin 3 on the ST goes to Pin 2 on the RS MAF BN/BU wire
- Pin 4 on the ST goes to Pin 4 on the RS MAF WH/RD wire
- Pin 5 on the ST goes to Pin 5 on the RS MAF BN/RD wire

Your car’s stock intercooler is a fraction of the size it needs.
Even with a stock vehicle, any time you step on the gas you are robbing yourself of horsepower, because your stock intercooler does not have the ability to cool enough air.
It  runs hot air into the engine, giving your turbo engine sluggish throttle response, acceleration and an increase in turbo lag.

TIME TO STEP IT UP!!!

The 2 key factors in how much power an intercooler will produce on your car are:

 Pressure drop!!

If your boost system is restricted it will create pressure drop, which in turn will heat up the charge.

By improving the pressure drop between the air filter and the turbo. Or by having a more efficient compressor wheel. 

Which allows you to run the same amount of boost in the intake manifold while having a lower turbo discharge pressure.

You can also reduce the pressure drop through the intercooler. By selecting the right size core for the job

 Temperature Drop!!

Getting the temperature of the intake air as cool as possible at the outlet of the intercooler.

This require a properly selected intercooler core, High flowing with the best cooling efficiency.

Properly designed end tanks either cast or constructed with the smoothest flow . Air scoops or  diffusers can help to force more air through the cooler core.

A high performance intercooler that reduces the pressure drop allows your engine to make more boost pressure at the throttle body without your turbo having to work harder than it was working with a less efficient intercooler.

In turn it  can allow your car to produce more boost at the throttle body with your  stock turbocharger.

A high quality intercooler core coupled to high flowing inlet and out let tanks can make a big difference to inlet temperatures and pressure differentials,