I know that the 02 sensor compares the outside air to the inside air, but how does covering it in tin foil cause it to lean out? What's the mechanism? Are you trying to "starve" the 02 sensor from outside air?
How does an O2 sensor work?
An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts. The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor *will not spend any time at 0.45 volts*. In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts.(1) Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution. The O2 sensor is constantly in a state of transition between high and low voltage. Manfucturers call this crossing of the 0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.(2)
(1) - This is *exactly* what I see in my car at startup, 0.45 volts => 14.7 exactly.
(2) - You are talking about doing something like this, but in a controlled way. Would there be a "pinhole" in the tin foil to allow for air exchanges? How about an external ion generator to spoof the 02 sensor ?
"Typical Ford cost cutting, my O2 sensor had no anti-sieze on it either. The O2 sensor wrap is working great on the 1st sensor. I'm installing some on the second sensor and will report FE on the next tank. I'm also considering wrapping the wire all the way to the connector. There is some sort of heat block sheathing on there already, but more couldn't hurt.
For those of you who are interested, wrapping your O2 sensors and wire with about 10 times or more with aluminum foil, and/or spark plug boot heat protectors gives approx 2 mpg. I have mine wrapped with aluminum foil and I'm using a DEI spark plug boot protector to hold the tin foil together. Copper or aluminum wire can also be used to secure the aluminum foil. I wrapped approx 6" of the sensor and wire which is about the length of the spark plug boot protector. I also had to cut a ring out of the cloth protector in order for it to fit over the huge connector. To remove the sensor wire from the clip, look to see which side is hinged, and use a flat screw driver on the clasped side, push in and turn. You can also simply pull the clip out of the hole it sits in. I'm considering using aluminum foil along the entire length of the wire, and removing the clip entirely later today, The foil should hold the wires further away from the exhaust which is what the clip is designed to do".
I took this from http://www.focusfanatics.com/forum/p...&pagenumber=10
The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.
The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It is hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail.
The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor output voltage will drop down to about 0.2 volts or less. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around .45 volts.
At first I thought I had a little hole in my 02 sensor, but now I think it is in the wire.