Quote:
Originally Posted by heatmizr
Redneck what you are saying is really intriguing...
Can you help me understand how crankcase vacuum is correlated with the pressure/vacuum in the head / valve stem area? - is it the same? If that is negative, wouldn't that keep the oil from getting forced thru the valve stem seal (which is under pressure from the turbos half the time, and maybe not when off-throttle).
And how / where is the PCV system attached, and what impact it has on the pressures above?
Trying to understand how your fix works.
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Sure, so lets start at whats going on at Idle:
In any engine, the throttle valve is nearly closed, in essence choking the engine and starving it of air. This is true if its an old 50's truck with a carburetor on it or a fancy-pants BMW with direct injection and a electronically driven throttle valve.
This creates a vacuum, different depending on the many factors but in general most engines are around -20 psi At idle when you intake valves open, the are subjected to this vacuum. When your engine is new and the valve seals are super tight, no oil can get sucked through. However when shes been around the block a few times, the seals wear and now can allow some oil to pass.
As an engine wears it also increases its blow-by: this is the amount of gasses that go around the piston rings and into the crankcase. These gasses must be vented or ideally recycled back into the intake as they are quite nasty and thus why all engines have some form a PCV or CCV system. Within these gases is trapped oil, so some form of a oil/air separator must be used. In the 50i, these are tucked under the valve covers. They kinda suck though as if they did their job my catch cans would be dry and our engines would not have the carbon build issues they suffer quite often from.
BMW PCV systems are notoriously weak. They tune them for about -1psi. Thats all fine and dandy when the engine is new, put 100K on it though and the increased blowby with worn valvestem seals and you get the messy smoking at idle syndrome.
The reason why this goes away at part or full throttle is the vacuum in the combustion chamber goes to 0 at full throttle on a normal engine and on the 50i will go to 8 to 12psi depending on the temp. This eliminates the oil coming down through the valve stems and thus why our cars are notorious for smoking at idle but not when we are cruising down the road questioning why everyone drives so slow (if yours smokes while driving, its probably clogged turbo oil lines or shot turbo seals)
Below is a pic of the 50i's CCV (it uses two of these, plus tubes that go to each intake manifold on the engine) I whipped up. Its s two stage system as under boost the PCV valve must close as to not pressure the crankcase and it switches to the force of the intake air running past the CCV pre-turbo inlet to pull in Crankcase gasses and still create a mild vacuum under boost.