Blow by occurs when piston rings loose their ability to seal adequately. They allow combustion gases to leak past them, pressurizing the crankcase. A pressurized crankcase causes problems such as blown seals, gaskets failures, excessive oil usage, and a loss of horsepower. Blow by is normal in high mileage engines.
Either that, or talking about when I lay the hammer down and blow bye some left lane hogging backmarker in his/her rolling chicane.
Perform a leak down test. I’m going to assume your next question is "What’s that?", and answer it-
A leak down test is a way of telling how well your cylinders are sealing. I perform a leak down test by pulling all the spark plugs, and then turning the engine over (by hand, which is a good reason for removing the plugs) and getting the most convenient piston at top dead center. Being at top dead center means both valves are closed. I screw a threaded coupler into the sparkplug threads. I plug the leakage gauge into the threaded coupler and the air line to the leakage gauge. I feed 90 PSI of air into the cylinder and watch the leakage gauge. The lower the leakage, the tighter seal the cylinder has. Above 30% and it’s time to figure out where it’s leaking. If I can hear air through the throttle body, the intake valves are bad. Hearing air theough the exhaust pipe means the exhaust valves are bad. Either of those can be fixed with a valve job. If I hear air coming through the PCV fitting, the rings and cylinders are no longer sealing. That means it time for a complete rebuild. The leak down test really should be performed on each cylinder because it's possible to have "a" bad valve or (less likely) "a" bad cylinder.
Alternatively, you can take a car to a GOOD mechanic and have them do the test.
Blow-by is way over-rated as a method for determining engine health in a MB diesel. Better test: how quickly does the car start when cold. If it starts quickly, then it has sufficient compression and the glow system and the injection system are all working like they should. Loss of compression, which will increase blow-by, will very slightly affect power and/or fuel mileage, not really worth messing with. When the car becomes difficult or impossible to start when cold, then compression is too low.
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