Trelinski,

Blow-by is high pressure gas from the combustion event on the power stroke, that makes its way past the piston ring/cylinder wall seal. When the car is relatively new and in good condition, there is little leakage past the piston rings and little or no apparent blow-by. As the car ages, depending on things like oil changes (frequent is better) and operating temperatures (running hot or overheating is bad), this seal can degrade faster or more slowly.

Pulling the oil fill cap off with the engine running at night, and shining a flash light across the openning has helped me see the blow-by better. If it is readily apparent without a light, or in daylight, and resists your effort to put the cap back on, the car is getting old. If the evidence of blow-by is not apparent or hard to detect, the car is in good shape.

Congratulations on the progress you have made in your selection process and good luck finding a jewel of the W126 model line! Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)