Originally Posted by Chris 850T
It gets it's reading from the MAP sensor which is usually located on the throttle body but can be located elsewhere. All this does is tell the ECU how much vacuum or boost is in the system so it can adjust fuel and timing tables accordingly. It also uses an intake air temp sensor to aid in timing adjustments. Also known as IAT sensor.
The MAP sensor tells the system how much pressure (vacuum) is in the intake system in a normal engine. I always think of the MAP as reading vacuum simply because it's easier on my mind but it's actually calculating pressure in an "engineers" mind. I'm not sure exactly what it does under boost in a supercharged or turbocharged engine.
MAP is manifold absolute pressure and it somehow measures the difference in pressure (vacuum) between the intake system and the ambient outside pressure and feeds that signal to the computer.
The intake and the crankcase are two separate systems and have nothing in common. The slight vacuum the PCV system is supposed to apply to the crankcase is not monitored by any sensor I know of. It's just there to pull fresh air into the engine usually filtered and supplied just after the air filter housing and then sucks it into the intake system to be burnt after it's picked up moisture and blow by from inside the engine.
I don't think the vacuum is strong enough to pull the glove in but you can also try a glove or balloon over the dipstick tube to get the same effect.
Cut and paste about boost sensor:
MAP sensors measure absolute pressure. Boost sensors or gauges measure the amount of pressure above a set absolute pressure. That set absolute pressure is usually 1 atmosphere (1 atm) or 14.7 psi
. This is commonly referred to as gauge pressure. Boost pressure is relative to absolute pressure - as one increases or decreases, so does the other. It is a one-to-one relationship with an offset of -14.7 psi for boost pressure. Thus a MAP sensor will always read 14.7 psi more than a boost sensor measuring the same conditions. A MAP sensor will never display a negative reading because it is measuring absolute pressure, where zero is the total absence of pressure (it is possible to have conditions where negative absolute pressure can be observed, but none of those conditions occur in the air intake of an internal combustion engine). Boost sensors can display negative readings, indicating vacuum or suction (a condition of lower pressure than the surrounding atmosphere). In forced induction engines (supercharged
), a negative boost reading indicates that the engine is drawing air faster than it is being supplied, creating suction. This is often called vacuum pressure when referring to internal combustion engines.
In short: most boost sensors will read 14.7 psi less than a MAP sensor reads. One can convert boost to MAP by adding 14.7 psi. One can convert from MAP to boost by subtracting 14.7 psi.