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Old 03-02-2012, 11:13 AM   #15
jdstrickland
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Join Date: Apr 2010
Location: Murrieta, CA
Posts: 6,722
My Ride: '94 325iC & '00 323i
Quote:
Originally Posted by Solidjake View Post
Yup, the sap. It heats up the o2 sensors
+1, sorta.

The noise is the SAP, but it does not heat up the O2 Sensors.

The CAT uses air and heat to convert fuel into clean air. When the CATs are cold they do not work very well so air is introduced into the exhaust stream. The exhaust stream already has air in it, and there is fuel, but there is no heat to speak of. The absence of heat is countered with more air molecules so the fuel molecules have more to bond to. When the heat arrives, the fresh air can be shut off.

The O2 Sensors heat up because they have internal heaters in them that are turned on. After the exhaust system heats up, the heaters in the sensors are switched off. I'm not sure of the timing of the heaters, but logic says the timing should mirror (more or less) the same timing as the SAP.
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