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Old 01-18-2013, 03:51 PM   #5
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Join Date: Apr 2010
Location: Murrieta, CA
Posts: 9,258
My Ride: '94 325iC & '00 323i
Originally Posted by glhx View Post
My SAP may have been bad for a long time and I not know it because i dont know what its supposed to sound like. I know ive never heard any distinct noises from that area while under the hood ever.

The SAP sends fresh air to warm up the cats....also sends fresh air to help burn the rich fuel when cold starting. This is where i could think they are related.

Did running rich for a long period of time on cold starts carbon foul the o2 sensor.....which is past its replacement date anyway. only speculating here.

my thoughts on this are that one or both of my precat o2 sensors are bad and went bad over time due to lack of SAP and wear and tear.

It could be the MAF....i found an app to measure MAF in g/sec. havent done this yet.
The SAP _only_ provides fresh air to help the CAT work efficiently while it is still cold. There is no other purpose for the SAP.

The CAT uses a chemical process and heat to help clean the exhaust and reduce emissions. The CAT does not work well while it is cold, but it works better if cold AND there is fresh air available as a substitute. Once the CAT warms up, the fresh air is not needed, until the CAT warms up the fresh air helps the CAT do what it is supposed to do.

The SAP has no bearing on your mileage. None. Not any. Not ever.
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