While BMW might not be the first Auto Maker to introduce Variable Timing to control engine valves, it is one of the first to offer infinitely adjustment across the rpm band. When it was introduced in 1996 the VANOS Unit only work on inlet valve providing better engine breathing with flat torque available from low to high rpm.
Earlier Leisure Drive did mention at one of articles on Automotive Technology Milestone the fact that in its inception the multi valve engine despite of wider rpm it did suffer torque loss at lower band rendering sluggishness at walking pace traffic, the ability of the car to climb a decent ascending road might necessitate downshifting unless the rpm is beyond 3,500 rpm.
This hurdle had become apparent for example a 1992 non VTec automatic Honda Civic; the 1,500 cc engine did not have low gear ratio selector, so upon climbing a hill at less than 3,000 rpm the Civic simply walk giving a loss of power impression.
The Advanced Technology solution on VANOS equipped BMW actuates the second inlet valve to be turned off at such low rpm band, allowing for optimal torque to be attained, and hence the car has sufficient power to out accelerate any climbing section. But unfortunately BMW VANOS Unit is notorious to fail around 60K km mileage.
And often the available rebuilt replacement has been found to also fail at 20k miles (32k kilometres). If your 1992-2006 BMW 6-cylinder cars suffers from sluggishness, bogging then surging at 3K rpm with uneven and erratic rpm transition, engine hesitations below 3K as well as louder idle often accompanied by intermittent idle rpm hiccups.
The car will no doubt have increased fuel consumption and giving out VANOS fault code read out. The culprit is definitely due to deteriorating piston seal O-rings effectively blocking the piston to actuate VANOS camshaft. An upgraded seal material would be a wise replacement, and more and more third party vendor could provide such replacement at less cost than the dealer. One of the best resources would be BMW forum and Beisan.
For some reason BMW only provide the replacement of the whole unit instead of merely the seal itself. There is a price to be paid for using Advanced Technology; that is why some folks are retaining dearly their reliable Classic Cars.
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