Blow-by, Compression, etcetera . . . .


Unable to sleep last night, because of my hard-starting OM615, I got thinking of old Gustaf Diesel who must have had many sleepless nights before he gave us the goodies we so much like and fiddle around with—and willing to spend endless dollars on it in order to make it run or keep it running! In finding a solution to my problem I must have dug up every hint in the Forum that would point me in a logical direction--blow-by in particular. Now, I'm no Gustaf nor even close to some of the real Dieselmen in the Forum, but it doesn't prevent me from thinking in a logical sequenceconcerning the problem of alleged blow-by—which for many is the deciding factor whether the engine is still good or a candidate for the scrap heap. Lets start with the fact that any propulsion engine in the world may it be gas or Diesel—outside nuclear—requires a liquid capable to ignite—gas by an electric impulse; Diesel by way of super high compression. Lets stay with Diesel: A piston moves up and down within a cylinder. To seal the piston to the cylinder wall is taken care of by rings to prevent the oil from reaching the combustion chamber. All three items—piston, rings, cylinder wall—will wear in time, to a point when the original tight fit becomes less and less through wear, and oil will reach the combustion chamber. There it is burned up, and the gases expelled through the exhaust where it is visible as blue smoke. QUESTION 1.) How is it possible that the oil sneaking into the combustion chamber ever be identified as blow-by? In my novice-thinking, the only way blow-by can occur is that the valve seats are leaking and some of the compression escapes into the overhead valve chamber where it mingles with the chain-splash and the lubricating oil in general—depending on how bad the valves leak determines the blow-by. QUESTION 2.) Am I on the right course or not? Let me assume I'm on the right course, a logical conclusion would be that a blow-by, by my thinking, would cause lower compression because some of the force that should be harnessed in the compression chamber is now exhausted through the filler cap or—when closed—through the ventilating mechanism. And touching on the subject of compression, my Workshop Manual (a MB original) states the following: If a drop in compression is noticed (rated value of 20-23 atü 284-327 psi drops to 13-15 atü 185-213 psi) it will be sufficient to install only a new valve turning device for each of the inlet and exhaust valve in the cylinder in which a drop of compression pressure prevails. End of quote. I have taken my time writing on this subject as a complete novice but with the idea in mind that it will encourage a discussion, and hopefully an answer from the many experts in this field within the Forum—and for myself finding the answer in getting my blooming engine started.