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Old 02-06-2006, 03:28 PM   #6
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Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
Posts: 3,308
Originally Posted by xclusivecar
To note: Finally had the 4th sensor replaced under warranty last week and immediately noticed the idle dropping rather quickly on cold start now. Before the car would idle at 1200 RPM for about 2 minutes until dropping to 800 RPM. After new O2 sensors were installed, the idle dropped to 800 RPM after maybe 20 seconds! Is this a matter of the sensors heating quicker than before and therefore telling the computer to relaxe the idle? This alone is probably where the savings in gas comes in!?!?!

Yes that's certainly part of it, but not the whole story. If the O Sensors aren't operating optimally, the electrical signal they generate is off-spec. The DME reacts in several ways simultaneously affecting the FI, Ignition Spark.

Heating elements with a reduced efficiency delay the onset of the Chemical reaction between the O in the exhaust and the Ceramic disc in the Sensor (the Ceramic must be at least 650F for this reaction to take place).

But, in addition to this, running cold longer allows combustion byproducts to condense on this Ceramic reducing the surface area it has exposed to the exhaust gasses and any unburned O.

It may sound insignificant, but Bosch states it useful life for Heated O Sensors as 60K mi., but only 30k mi. for Unheated ones, pretty big difference for just having a Heater Coil in one, while the other relies on the Exhaust Gas Heat alone to raise it to operating temp.

Once this inaccurate signal degrades to a certain point, the DME goes into a Default MAP which runs Richer than normal because it wants to insure against running Lean, so more fuel is actually used. According to Robert Bosch, this can amount to $100/year for the average driver.

Happy Motoring!...Jim'99

Last edited by MNBoxster; 02-06-2006 at 03:32 PM.
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