High oil pressure: cap blows off!

Happy Sunday, all. I�ve been experiencing high oil pressure readings on my 996.2 C4S and had assumed it was just a faulty sensor/sending unit, since there are so many posts about that from time to time. But recently my oil cap has been popping off after a bit of driving. Fortunately it stays in the engine bay so I can put it back, but then after more driving, it pops off again. Clearly my oil pressure is genuinely high. I plan to do an oil change today, thinking that it may be as simple as too much oil. I�m sure I�ve topped up a bit because of low readings on the oil level indicator, but perhaps the indicator itself is inaccurate and I�ve overfilled it. This seems like the Occam�s Razor approach but is there something else I should be thinking about? Thanks.

Well, thats not from high oil pressure, as thats crank case pressure thats blowing off the oil cap. Not even sure how much pressure it takes to blow off a cap thats screwed on,,, seems like a lot... Don't drive it like that imo. Should have slight negative pressure in the crankcase, around 5-6" of water column iirr. Could have AOS failure/malfunction..? But most of the time aos failure causes way to much negative vacuum not positive pressure, and sucking oil in the intake creating a hydrolock is very possible when aos fails. Maybe a combination of excessive cyl blow by and aos not venting to intake...? Hopefully Porschetech3 our resident crank case venting expert, or another expert will chime in here and shed some more light on whats happening... Anyhow you've got a problem I wouldn't continue to drive with, could blow out a engine seal.. or worse... jmho

What failed is the positive crank case ventilation system, known as PCV valve in other cars. In Porsche 996, that is the AOS system You need to check, find, and replace that valve. I have seen one other case where the owner DIY changed his throttle housing from stock to a GTx housing which had a different fitting for the ventilation feedback line. He made a sharp bend in the line to make it fit and it became a block instead of a vent. The crankcase built up so much pressure that the cam shaft plastic oil plugs got popped off resulting in a massive oil leak as he was driving. DON'T drive it like that without fixing it.

I wondered about AOS but I have no smoke. When the AOS on my 986.2 Boxster failed, it smoked massively. I thought about crankcase ventilation, which made me think PCV, which then made me think AOS. I was unsure whether there was a separate PCV or whether it was in/part of the AOS. If it's part of the AOS I guess it makes sense that the PCV can fail without the typical AOS symptoms.

It seems this is a different failure from your Boxster. If the diaphragm rips, you get huge vacuum in the crankcase from being open to intake vacuum. I think this is where the smoke comes from, sucking in too much oil laden air from the crankcase. In the current case, it seems like it is plugged and there is no interaction between the crankcase and the intake. Since the crankcase can't give away the pressure created from blow by, and such, it just pops the cap, or caps, off.

Leave the oil cap off and crank the engine, put your hand over the oil fill tube and you should feel a slight vacuum , if using a manometer if should read 4-6 inwc If there is a high vacuum the flow regulator has failed. The flow regulator is integral to the AOS in the factory design. In the UAOS design the flow regulator is separate and is moved to next to the throttle body..( the Cyclone air-oil separtor remains in its location.) If there is no vacuum the crankcase vent system is plugged somewhere. ( the UAOS is a complete system).. As mentioned , don't drive the car like this. I'm surprized you haven't blow out a cam plug.... If you have to run the engine, leave the oil cap loose/off to prevent pressure build-up... I suggest getting a manometer and learn to use it or taking it to a Pro..

Thanks for the responses. I have to admit I had to look up "manometer," so suffice to say I don't know how to use one. Perhaps my level of ignorance becomes a threshold issue telling me I need to get a tow to a shop. But before I resign myself to that, is it safe to assume that the issue is indeed in the AOS and that replacing the AOS would resolve the crankcase pressure issue? Or might there be another culprit causing the high pressure? If I can determine it's the AOS, I'm probably willing to use this as the opportunity to convert to the LN Engineering UAOS model and just work on it over a series of evenings. If there's a bunch of diagnosis work that needs to be done, it probably needs to be done by someone more experienced than me. I'd prefer to "DIY" it even knowing it's a fairly significant undertaking. I'm not afraid of the labor; I simply lack the experience to diagnose.

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Yes, it very safe to assume that installing the UAOS will solve the problem...Plus the UAOS has numerous other advantages over the factory design, and even cost savings over the long run...

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You�ll get to miss having to pay for an engine drop and r/r of a brand spanking new Porsche AOS that fails out of the box. Rrrrrr!

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Don�t forget about some folks having to do it twice or thrice. They will not get good at engine drops if they switch!

A manometer is simple to use, and fairly cheap as well.