[ferocity02]

Today I pulled off the oil cap while the engine was running and noticed a rather significant amount of vacuum inside the crankcase. As I was removing the oil cap I could hear it hiss as air suck past the cap into the engine. I know a little vacuum is normal, but this seemed excessive, way way more than my N54. The idle also stumbled quite a bit in the process.

Is this normal? Bad PCV?

Is there a good way to measure crankcase vacuum? Is the a proper spec/range?

[weehe126]

Possibly normal, but to check you PCV valves, there is a nipple on the top of the valve cover. Remove the plastic cap and see if the nipple is pulling in air. I use a tissue, others use match smoke. If it is pulling in air you have to replace the whole VG.

[ferocity02]

Originally Posted by weehe126

Possibly normal, but to check you PCV valves, there is a nipple on the top of the valve cover. Remove the plastic cap and see if the nipple is pulling in air. I use a tissue, others use match smoke. If it is pulling in air you have to replace the whole VG.

Do you have a picture of where this nipple is located?

If it sucks in any air at all does that mean the valve cover needs replacement?

[Welcome to NBA Jam]

There is a spec out there somewhere and you can actually measure it with a special tool. I have no idea what it is though. The main concern people have with excess crank case pressure comes from a rupture in the PCV membrane. If you take the beauty cover off, it's located in that big cylinder thing that shows an electrical warning on it.

[The Silver Slammer]

https://www.e90post.com/forums/showthread.php?t=1197666

[bbnks2]

Excess pressure can rupture the membrane in the PCV valve but you would have LESS vacuum in that type of failure where ambient air gets pulled in through the pressure regulator. Vacumm would not be able to build in the crankcase.The PCV check valves can also fail allowing too much intake vacuum on the crankcase. You would not be about to use the "smoke test" on that type of PCV failure.

Strong vacuum sounds normal. The car stumbles when you take the oil cap off because N55s are MAF regulated (the sensor on your intake) not speed density like N54s. When you open the oil cap un-metered MAF enters the engine and the car struggles to keep AFR in check as the car leans out due to excess air entering the system.

[Pladi]

It sounds comprreterry normal

[vespa]

Originally Posted by bbnks2

Excess pressure can rupture the membrane in the PCV valve but you would have LESS vacuum in that type of failure where ambient air gets pulled in through the pressure regulator. Vacumm would not be able to build in the crankcase.The PCV check valves can also fail allowing too much intake vacuum on the crankcase.

You are correct that a ruptured PCV regulator membrane allows outside air to flow into the crankcase, but the major effect is that the regulator no longer closes properly to limit exposure to intake vacuum which dominates over the small outside leak so the end result is that the crankcase has excessive vacuum, hence the whistling crank seals.

Also, the check valves only serve to let air out of the crankcase. I haven't heard of any failures, but if either check valve were to leak, get stuck open, or stuck closed, the result would always be excess positive pressure in the crankcase, not vacuum.