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Old 11-26-2012, 03:28 PM   #32
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Join Date: Apr 2010
Location: Murrieta, CA
Posts: 9,243
My Ride: '94 325iC & '00 323i
Originally Posted by TEAMAFLCON View Post
Is this the sound?


Wait for the engine to be cold -- over night, at the end of your regular work day, whatever it takes -- and raise the hood and start the car.

At the front of the engine, immediately behind the right side headlamp, mounted to the shock tower, there is a large motor that makes a sound like a vacuum cleaner for about 2 minutes following Cold Engine Start.

During cold engine operations, the CAT does not convert the exhaust to clean air very well, so they pump in fresh air so that the process is more complete for the first few minutes than it otherwise would be. Basically, the CAT needs oxygen and heat to scrub the smog out of the exhaust. Before there is heat, they can scrub lots of the pollutants out by simply introducing extra fresh air, this is what the Secondary Air Pump is doing. After about 2 minutes, there is enough heat in the exhaust for the CAT to do its job all by itself, so the SAP can be turned off.
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