You guy are putting the aluminum foil on the OUTSIDE of the O2 sensor and cat area to retain the heat right?
I figured that since I am running at a slower speed most of the time and buring less fuel than the standard xB I would need to keep the exhaust warmer to keep the Cat working so I partially wrapped my header by putting foil over its heat shield - not directly on the header pipe. Seems to be standing up very well after more than a year.
The aluminum foil was sounding better but now I am trying to double check the logic behind it and things aren't adding up. Hopefully someone can help me see how the foil is supposed to work in light of the way that o2 sensors work.
So o2 sensors work by looking at the difference in oxygen levels between the exhaust stream and the engine compartment (they are vented through the wire or through the housing around the sensor).
I am going to make up some numbers here-
Lets imagine that the actual oxygen concentration in the engine compartment is an arbitrary number like 10. Looking at the exhaust stream, imagine that ideal combustion is a 3 and a rich mixture would be 2 (less oxygen) a lean mixture would be 4 (more oxygen).
So, if the sensor compares the differences between the engine compartment= 10 and a lean mix= 4, that would be a difference of 6 (so its output would be less than .45 volts).
A rich mix would have a difference of 10-2= 8 (output over .45 volts).
So, if I reduce the amount of oxygen to the outside of the sensor by wrapping it with foil etc., lets imagine that I have lowered o2 to that end of the sensor so that it always sees a level of 8 (rather than the unwrapped value of 10). So now a lean mix would have a difference of 8-4=4 (very low voltage output) and a rich mix would have a difference of 8-2=6 (output still less than .45 volts).
This rich mix (in reality) would be interpreted by the wrapped o2 sensor as a lean mix because it would be outputting less than .45 volts and the computer would compensate by richening the mixture. So wrapping the o2 sensor could be a bad thing for mpg because it could trigger a richer mixture.
I see how the foil could help keep it warm though.
Inside outside does wrapping the sensor on the outside affect the O2 level of the air that reaches the sensor?? I don't think so! If the air is getting into the sensor via the connector contact it would take a lot of aluminum foil and maybe some silicon sealer to keep the air out. Wrapping can protect it from water and dirt and I don't think it would trap moisture near the sensor because it gets so hot the water would boil off in seconds of starting the engine. Good article on the O2 sensor except for the "voltage to flow" wording - voltage doesn't flow, current flows and in fact it probably produces voltage as the mixture gets richer or its resistance goes up increasing the voltage drop across it from the current supplied from the ECU.
JanGeo- That makes more sense- that the only benefit of the aluminum foil is to retain heat.
If it blocked any o2 from the outside of the sensor (which would be unlikely if o2 came in through the wire), it would seem to have a richening effect (worse mpg) than a leaning effect and helping mpg.