Blow by is pretty much defined as "air" that escapes past the piston rings during the combustion (power) stroke causing excess pressure in the crankcase, so I'm not sure what you're asking.

Valve seals might give you a burning oil problem, but they aren't going to cause blow-by unless you have exhaust valves that aren't letting exhaust gas out.

You might have a stuck PCV valve. Check to see if you have oil where it taps into your air intake tubing. To check for a blocked cat, just remove your upstream O2 sensor and see if that lessens your valve cover pressure.

From the "super windy open oil cap" description, though, it sounds like you probably do have an excessive amount of crankcase pressure, which is usually a cylinder-sealing problem, and at idle the PCV is pretty much closed anyway, so that kind of rules it out as a PCV problem.

What weight oil are you using? If you're using a super heavy weight oil, you could actually be making the problem worse, since the oil will act against the spring tension of the sealing rings to the point where they become weak & ineffective.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.