Positive crankcase pressure

Hey, As I understood, there should be a slight vacuum in the cranckcase of the M96 engine. If the AOS fails, this vacuum increases a lot. BUT what if there is a slight positive pressure in the crankcase? If I remove my oil filler cap, the air is coming out from the filler hose and the engine idle is also different... Additionally my PSE is also in loud mode, both sides... Could be these two problems related to each other? I have ordered the AOS from Pelican Parts store, intended to change after engine drop, Just curious what else should I check? No fault codes, even with Durametric, runs and drives perfect! 2002 996 C4S, Thanks for your help, Cheers, Tom

Crankcase pressure occurs when a vehicle's breather tube is obstructed (bad AOS or blocked breather tube), the pistons or valve oil seals are worn or defective, the piston crown is cracked, the gaskets are defective or the vehicle has excessive exhaust back pressure. Usually results in excessive oil consumption and poor running. I'm really surprised you are not smoking and it runs and drives perfect.

PSE. Do you have a switch on the dash? does it work? someone may have hacked the relay to always be in the one position and only turn off with the switch. Or they could have jacked with the vacuum lines controlling the PSE valves. There is some good info on how the PSE operates on here. Do some searching and you will find all you need to get it to how you want it.

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The idle changing with the oil filler tube cap removed is normal. The open oil filler tube is an intake air leak. Unmetered air -- air that has none passed by the MAF -- is being introduced to the engine. You sure air is coming out the filler tube? Air flowing into the open tube can feel like air is flowing out. (I get the same impression when checking radiator fan action. Holding my hand in front of the radiator duct opening and with the radiator fan running at first makes me think the radiator fan is blowing air out the front opening but then I recognize what is going on. Still I then move my hand to after the radiator fan to where the radiator fan air exits and confirm the air is blowing out. And that the air is about the same temperature as the other side.) I would not advise you to put your hand over the oil filler tube opening but you can put the cap on the tube without turning it to tighten it down. If the cap floats or blows off then that's just wrong. My guess is it will remain in place due to the low pressure present. If in fact the crankcase is under positive pressure at idle more than likely that's some kind of AOS failure. While there are other mechanical engine problems that can result in excessive crankcase pressure there should be other symptoms/signs of a problem and you report none.

Take the cap off and put a sheet of paper on the filler neck, there should be a slight vacuum that pulsates with the pistons. So the paper will flutter up and down.