I am seeing substantial blow-by on a 190e 2.3 78k engine. It is suggested that it could be valve seals. piston rings (stuck) or pcv unit. Does anyone have information or guesses as to what and where to check for the most likely causes for a 78k engine. I will say that the blow-by from an open oil cap will give you a new hairdo or g-force experience. The engine seems to have plenty of power although with intermittent rough idle which I have attributed to a relay or sensor somewhere . No engine smoke or exhaust problems other than the suspicion that my cat converter is restricted. All is too new looking for any real problem if that is of any criteria.

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.

... could be reaching way out there, but has the car been setting for awhile and not started... say for a year or so? Sometimes if a car will set for a long time without being started, it will smoke for awhile.

My old 1984 190D smoked like crazy when I first bought it, but it had not been driven for a couple of years as the PO had deceased and his wife parked it in the garage. ALL fluids and filters were changed, but it still smoked.... but after about 250 miles, all smoking went away... it was a diesel though, so not sure how much a gasser would ease the smoking in that amount of time.

Most importantly, what color is the smoke and what does it smell like?

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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.

Would suggest running a quart or two of automatic transmission fluid in your engine for a couple of hundred miles...then change oil and filters...this oil is very heavily laden with detergent compounds and will help clean out sludge/varnish that may have accumulated while sitting...very common problem..also attacks fuel dist., fuel pumps, fuel filters etc etc...so you rough running might be related to dirty fuel systems, and the blow by to stuck sealing rings on the piston. Dried out valve stem seals might allow this if your exhaust is restricted as mentioned...no blue exhaust smoke is also a good sign as normally excessive blow by will result in burning oil and visible signs thereof.
Also check the valve cover vent valve which should be found in the breather hose that goes from the valve cover to the air filter housing...not sure where it is located on the 8V's, but it should be in that breather hose assembly. A plugged vent will force all excessive valve cover gasses out the oil fill port instead of into the filter box.