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I recently installed the MHD stage 1 on my 2013 135i (N55) and i immediately noticed the rear main seal squeal sound after the flash. I put the stock tune back on and the squeal went away. So I researched everything and found that the squeal is a symptom of PCV problems. I performed the flame test on the PCV valve nipple and verified that the PCV membrane had likely failed. I found it strange that there was no squeal on the stage 0 (stock) tune, but there was loud squealing every time I came off the throttle with stage 1.
My hypothesis is that the change in valve timing from Stage 0 to Stage 1 results in a reduction of positive crankcase pressure. On stage 1 the engine does does not create enough positive crankcase pressure when off throttle to balance the amount of unregulated vacuum being pulled through the broken PCV valve. This extra negative pressure pulls air in through the rear or front main seals and makes the squeal sound off throttle.
So I have replaced the PCV valve yesterday, rather than the whole valve cover, via a 15$ Ensun Amazon replacement and some JB weld. This morning the car ran fine as usual on stage 0. I will test stage 1 after work and report back here about the squealing.
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