Ok, I'm gonna try and clear this up for everyone. I'm a mechanic by trade, but I really could just be anyone so you can choose not to take my word for it. FAST-XR was pretty much on the money. Later model cars have engines fitted with Positve Crankcase Ventilation Valves (PCV). What these do is allow for the changes in volume within the crankcase due to the engine oil heating up/cooling down (expanding/contracting) thus changing the volume of air within the engine. The PCV allows air to pass in and out of the engine when the volume changes due to what I explained before so there is not any undue pressure build up/vacuum resulting in power loss caused by this pressure/vacuum acting against pistons on a downward or upward stroke. So when the volume changes, the air travels through the PCV and out through the breather (on top of your rocker cover) and is usually plumbed back to a part of the intake (anywhere after the air filter). The reason for this, when the engine crank is rotating, it churns up the oil as it spins. This causes some of the oil to change state into a vapour. And remember no engine is perfect so along with this vapour, there will be blow-by gasses which will include carbon monoxide, sulphur dioxide, and unburnt hydrocarbons (fuel) all off which come under the 'emissions' tag. All this comes out the PCV. So..... when the blow-off valve opens and if its vented to atmosphere you are allowing emissions out that have not been recirculated back through the engine and passed through your catalytic converter. It has nothing to do with what kind of intake sensor there is on the ford because all air coming in is measured so close to the intake manifold, the ECU can accomodate for any changes. This is why everything is plumbed back before the sensor, so it is able to measure everything as accurately as possible. Hope this has been able to clear things up for everyone.
