Hi Blow by is created at the time of power stroke. At the time of power stroke, piston is pushed to downwords. Because of wear of piston rings & block, exhaust gases are leaked in crankcase ,when piston is pushed to downwords.These escaped exhaust gas through the block wall creates negetive pressure in crankcase so we face power loss. Blow by can cause power loss. See the video http://www.youtube.com/watch?v=PvDvcl29ZqMCheck wheather your MJD has blow-by.