What really goes bad? I know there is oil, pressure, solenoids and gears invovled. So I have to ask, when a unit needs replacing, what was it internally that "went bad"?

So we're all replacing our vanos but don't know why?
I really want to know!

From Pelican Parts's website:

The VANOS unit is hydraulically operated using pressurized oil from the engine. As such, the oil line that feeds the VANOS unit often begins to leak from age. It’s very common for this line to weep oil and leak it on top of the engine. To replace it, you will need to remove the intake manifold first in order to gain access (see our intake manifold removal article (http://www.pelicanparts.com/bmw/techarticles/101-Projects-12-Intake-Manifold/101-Projects-12-Intake-Manifold.htm)). Once the manifold is off, removal of the line is as easy as disconnecting the two banjo bolts on each end, and removing and replacing the line. Use new aluminum sealing rings when you install the new line.

Also accessible is the VANOS unit solenoid. This unit is responsible for actuating a valve that opens and allows pressurized oil to flow into the VANOS unit. To remove and replace this solenoid, you will also need to remove the intake manifold, as the electrical connector for the solenoid is hidden under the manifold. Be sure to use a new O-ring on the VANOS solenoid when you install its replacement.

http://www.pelicanparts.com/bmw/techarticles/101-Projects-18-Vanos-Line/101-Projects-18-Vanos-Line.htm

From Pelican Parts's website:



http://www.pelicanparts.com/bmw/techarticles/101-Projects-18-Vanos-Line/101-Projects-18-Vanos-Line.htm

Awesome, thanks. Solves the leaky vanos question but it leaves the question, why would you need to replace the whole system for ~$1500.00 and when replacing, what does that entail? Certainly the leaky oil line can't be a $1500 repair.
BTW, would you have excessive oil in the intake if you had a leak? I think some oil is normal, no?

www.drvanos.com (http://www.drvanos.com) .. no longer $1500..

usually the internal gears get gummed up with oil and sludge, then won't move smoothly.

most of the VANOS fault codes I've seen are "VANOS mechanically jammed". or "VANOS - end position not reached". both of which relate to the gears jamming, or not operating smoothly.

Thanks for the details everyone, jamming gears, really didn't think that was the case.

As for drvanos.com, he is only talking about the I6. What about the V8's with vanos, is this not as much an issue?