The rubber tube ontop of the valve cover next to the oil filler cap is where crankcase fumes (blowby) are pulled into the intake from. Excessive blowby is when exhaust gases leak into the crankcase (usually past worn rings/cylinders/pistons) enough to cause problems. Some blowby is normal

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.