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Originally Posted by andrewwynn
I always forget n62 no throttle plate. It does create vacuum though something has to pull in the air/fuel mix :-).
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Understood, yeah. I get that. It's a more complicated system than I realized. That said, and for future searches, found this explanation ( https://www.xoutpost.com/bmw-sav-for...lue-smoke.html) of why a vacuum leak and/or failing PRV/CCV/PCV can cause both smoking and a lean mixture:
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A falling piston creates a vacuum in its combustion chamber. The intake valve opens, passing this vacuum state on to the intake manifold. The intake manifold would become a complete vacuum rather quickly if the plate on the throttle body remained closed. However, the throttle plate is typically open, to some extent, as it controls the air portion of the “air/fuel” mixture that keeps our combustion engines...combusting. The vacuum generated by the falling piston allows metered air to flow past the throttle body, through the intake manifold, and into the expanding combustion chamber.
Beyond the throttle body is a tube connected to the Mass Air Flow (MAF) sensor. A sensor that constantly calculates the amount of air being pulled through the air filter, past the MAF sensor, past the throttle body, and into the intake manifold. The MAF is insanely accurate; to the point where using a throttle plate on a cable becomes a laughable, and turbulent, way to control the flow of air into the combustion chambers. Enter active intake and valve timing (for another time). But, the point of this story is to demonstrate that the throttle body, once the car is warm, is not really used to control the amount of air entering the combustion chambers but, instead, it is used to maintain a constant vacuum or “demand for air” from the MAF. It remains open most of the time, especially when the MAF sensor is telling it that it is drawing in a very specific amount of air. The tube between the MAF and the throttle body contains several possibilities for air to “leak,” unmetered, into the intake. This is a problem because the computer expects the specific quantity of air that the MAF told it to expect and any deviation will cause an imbalance in the “Air/Fuel” ratio…this imbalance chemical reaction leads to unreacted fuel being pushed out your exhaust…fuel that could have been propelling you forward (quickly!). The computer can try to compensate…but it cannot measure what it can neither detect nor predict. This source of unmetered air can be air entering through a disconnected or torn vacuum line, a loose hose clamp, or crack, on one of the intake tubes, or the CrankCase Ventilation (CCV) system.
A lot of vehicles use a CCV system has a valve that is held closed at atmospheric pressure and only opens when internal pressure exceeds atmospheric pressure: the PRV is briefly pushed open, releasing the excess pressure from the crankcase, closing, and the mechanical process starts over. Essentially, BMW uses this system as well; however, BMW’s PRVs default to “open” at atmospheric pressure. This is significantly different because the system BMW uses requires a constant vacuum in order for the PRV to remain sealed; where a traditional system remains sealed until there is excessive pressure. Therefore, if the BMW system cannot establish a vacuum (due to a unmetered air entering the crankcase and/or intake); the PRV cannot close and it cannot prevent oil from entering the intake manifold. I assume BMW had an issue with the pressure generated by the alternative system and, instead, chose a system that requires there to be no pressure to be closed and the presence of any pressure is immediately escaped from the crankcase.
BMW uses the intake’s vacuum to draw unmetered air out of the crankcase until the crankcase itself has so little pressure that the weight of the atmosphere can push the Pressure Regulating Valve (PRV) closed. This works great….unless there is a vacuum leak in the crankcase. As explained in earlier posts, the leak destroys the vacuum by equalizing the crankcase’s pressure with atmospheric pressure. The presence of atmospheric pressure, or any pressure, in the crankcase is enough overcome the PRV, releasing it to its default “open" position. This means that a crankcase vacuum leak allows unmetered air to flow past the PRVs and into the intake…it also means that splashing oil has an unobstructed path, through the same PRV and into your intake and combustion chambers.
We typically only see a cloud of smoke after the car has been warmed up and sitting idle for a prolonged period of time. Why? A cold car starts in “open loop,” a startup sequence that runs more or less based on memory, rather than sensor input. After the car is warm and the thermostats start functioning, the car *can* enter “closed loop,” a state where the car runs based on its sensors and less so on memory. The smoke issue tends to arise when the car enters Closed Loop because the Open Loop operation, uses more fuel, more vacuum, etc.. The added vacuum created by Open Loop’s sensor-ignorant operation is typically enough to overcome a typical vacuum leak…allowing enough of a vacuum to exist that the PRV is pulled closed.
Once the car enters Closed Loop operation, the throttle body relinquishes its control of the air flow to the active intake and valve timing. The finer tuned control of the air flow allows us to enjoy more power and better fuel efficiency…but this also means that it needs less air (and less fuel) to produce the same amount of power. Not needing as much air means that the throttle body doesn’t need to create as much of a vacuum…but less vacuum means that the crankcase vacuum leak can produce enough pressure to hold the PRV open. We typically only see a state of such low vacuum while at idle.
Unfortunately for us, the weak vacuum is still high enough to draw oil into the intake; but not quite high enough to pull the oil around the active intake and into the combustion chambers; at least not until you push the accelerator. Everything behind you disappears…and not in the typical “this is why I bought the V8” euphoria but, rather, a “Go-go-gadget smokescreen” kinda way…the humiliation typically only reserved for those that are lame enough to use go-go-gadget references in their posts. When you hit the gas pedal: the vacuum gets stronger, PRV is held closed, and the oil that made its way into the intake is drawn up and around the active intake channels and directly into the combustion chambers.
SIDE NOTE: The air entering the crankcase contains water vapor from the atmosphere. The PRV is the lone exit from the crankcase; its a very small opening leading to the intake manifold, the source of the vacuum. This tiny opening forces the water vapor and oil to mix before entering your intake...and some of it collects on the PRV itself. When it gets really cold, this cheese can freeze and seal the opening. The blow-by gases can create extreme pressure and in the crankcase, not being able to escape through the PRV nor through the tiny vacuum leak, the excessive pressure can cause the PRV to tear. When the PRV tears, you hear whistling as even more atmosphere is drawn into or blown out of the crankcase. At this point, you will need to replace the PRVs…but just replacing and cleaning the PRVs will not prevent the same situation from happening again. One would need to resolve the issue of how the air (and water vapor) is getting into the crankcase to begin with: a vacuum leak in the crankcase. The same issue, sometimes to a lesser extent, as the smoke cloud.
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This literally is explaining SO much of the issues I've had, including the "cheese" on the PRV. I also wonder if that froze, which is why I'm having so many issues in this weather. I need to get this thing to pass it's inspection in the next few days, but then I think I'm going to dive into some serious vacuum leak hunting. Pray it's not a valve cover gasket.......
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