Although not a "miracle" instrument, your oxygen sensor can tell you a lot about your air/fuel ratio. This is not an absolute instrument for it is only efficient at 14.7:1 air/fuel ratio and the exhaust temperature itself can changes the readings (mv's) by a few tenths! We've found that the 3-wire (heated) 02 sensors are more accurate than the 1-wire (non-heated) 02 sensors. We've also found that the heated 4-wire 02 sensors are more accurate than the 3 wire 02 sensors. Note: the 1 wire 02 sensors will not usually heat up enough from the exhaust temperature to work at idle and low rpm. If you are using headers, the 1-wire 02 sensor will sometimes not even heat up enough to work at cruise rpm's. More about that later - read on.
At idle and under .75 throttle the 02 sensor mv's should switch over and under .45mv's (14.7:1 air/fuel ratio) several times a second. Here, the computer is switching the air/fuel ratio from lean to rich to lean to rich, etc, etc. It does this for maximum catalytic converter efficiency and to make sure the average a/f ratio is at the proper 14.7:1 . Again, at idle or under .75 throttle, if it constantly reads over .45mv's you are running too rich - If it constantly reads under .45mv's you are running too lean.
At WOT the readings should not switch back and forth. The computer does not adjust to the 02 sensor signal anymore. You should now be getting a somewhat constant reading over this. Now, because the 02 sensor is not very efficient in reading exact lean or rich a/f ratio's the following chart should be used as a guide but not for exact information. With that said, naturally aspirated engine's typically make the most "safe" horsepower around 13-12.5:1. Super/turbocharged engine's "safe" horsepower is around 12:1.
Example: we've tested some naturally aspirated vehicles that ran best with with .7--.75mv's and some others that ran best with .8-.85mv's while achieving the same air/fuel ratio of approx.12.8:1. The difference was that the oxygen sensors produced somewhat differing voltages.
1) The first type is the single wire (non-heated) type. This relies on exhaust heat to bring it to the 650 degrees F. the materials need to produce voltage from the difference of oxygen content inside the exhaust system vs. outside of it. When there is not enough exhaust heat to raise the 02 sensor to approx. 650 degrees - as is cold start, low cruise rpm, headers and/or cat back exhaust, absence of catalytic converter, etc - the computer will rely on it's chip programming. This is no where as efficient as when the computer reads the 02 sensor signals and adjusts the fuel trim for proper air/fuel ratio. This is the first generation of oxygen sensors used by manufacturers and can be found on most GM Tbi vehicles. A few models in the last 1-2 years of GM Tbi production came with the more preferable 3 wire (heated) 02 sensors which are discussed just below.
2) The second type of 02 sensor is the 3 wire (heated) that is built with it's own internal heater. This heater typically only takes 10-20 seconds to heat the 02 sensor to it's proper operating temperature. Since this is wired to an ignition switched circuit the 02 sensor will now be on from almost the instant you start the engine to the instant you turn it off. Fabulous! Emission laws can work in our favor.
- Black wire is the signal wire
- 1 white wire is the 12v ignition switched wire
- 1 white wire is the ground wire.
- *** The heater is not concerned with which white wire is used as + or - , it is just concerned that 1 is used for + and the other - to complete the circuit. This makes it very easy to retrofit a 3 wire oxygen sensor in place of the very inefficient 1 wire sensor.
