breathers in efi valve covers=vac leak?

[PKRWUD]

I had to read this three times to figure out what was being asked.

Originally posted by DirtKing
alright...so if I had a breather in each valve cover, NO line to the throttle body and was running a pcv valve in the intake??? Under pcv conditions wouldn't the breathers vent IN not OUT??

Let me explain it as simply as possible. Air, from outside the vehicle, is drawn in through the air filter and is then metered by the MAF while it continues it's path to the TB. When it gets there, it's got a choice. It either goes into the engine via the TB, or it enters the crankcase through the hose that goes to the oil filler tube. It goes through the engine and exits out through the PCV valve at the back of the engine. From there, it continues to it's new home, the intake manifold, where the vacuum is that has pulled it along it's journey. It proceeds to the combustion chamber, has a real hot time, and ends up in the exhaust pipes.

The reason this works is because the MAF metered it before it went on it's journey, knowing that it would eventually end up getting to the combustion chamber. If you were to make a change in this system by adding a breather, then the air would get into the engine, and eventually the combustion chamber, without having been metered by the MAF. The result would be a vacuum leak.

If you must have breathers, the only way to do it that won't create a vacuum leak is to plug both the port on the tb where the air headed for the fill tube, and the vacuum port on the manifold that the PCV hose goes to. If you skip either one of these, the result will be a vacuum leak.

Get it?

Take care,
-Chris