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The Vanos system found in BMW engines is a great feature, keeping your power curve smooth and responsive especially in lower to mid RPM range. What usually goes unnoticed is the gradual loss of power within this range as the vehicles age due to slowly failing seals within the Vanos unit. The way the Vanos system works is based off of oil pressure triggered by a solenoid that allows for a small range of timing degree adjustment, this is where the power curve is affected. When the seals begin to fail (all of them do) there is less pressure to advance the timing reducing the power in that low to mid range. Because this happens slowly its very hard to notice a difference until you refresh your Vanos unit, then you easily realize the response you've been missing.
The main reason all of the Vanos units will eventually fail is because the material used for the sealing rings from the factory is not designed for that type of environment. When this happens the only option that BMW leaves you with is buying a new and expensive Vanos unit with the same sealing rings that failed in your original unit.
There are a number of symptoms that will plague your car as a result of these failing seals such as erratic idle, engine hesitation, stalling, and increased fuel consumption to name a few. If you've been chasing down any of these symptoms with no fix then this could be your issue.