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O-2 Sensors and use as a tuning/diagnostic tool
By Paul Ruschman - 2/28/99

blank50.gif (825 bytes) The O-2 sensor is used by Automobile manufacturers to monitor the exhaust gases of the internal combustion engine. The O-2 sensor uses two different exotic metals that react, (produce a voltage) in the absence of oxygen. The ECU of modern EFI systems uses this voltage to reference the air fuel mixture. Then the EFI can compensate, in small degrees, to keep the air fuel ratio where the programmers of the EFI ECU want the mixture.

When Fuel and air burn, certain amounts of Oxygen are left, depending on the mixture. The mixture that burns all the fuel is called "Lambda" or "Stoichiometric". "1" Lambda is the complete burning of the fuel or 14.7:1 Air Fuel ratio. (Air fuel ratio is measured by the amount of weight, or mass. I.e. 14.7 lbs. of air for every 1 lb. of fuel.) Anything less than "1" Lambda is a rich mixture and more than "1" Lambda is a lean mixture. I.e. "1.3" Lambda is lean, more air than oxygen.

The O-2 sensor produces a voltage once exhaust gasses heat the tip of the O-2 sensor more than 600 Degrees F. The O-2 sensor will produce form 0- to approx. 1.1 volts. Roughly .48 volts is Stoichiometric, 14.7:1, or Lambda. Maximum Power is achieved with a slightly rich mixture of approx. 12.8:1 or .9 Lambda. Minimum fuel consumption is best achieved with a slightly lean mixture of 15.5:1 or 1.05 Lambda.

This can seen on a quality Digital multi meter such as the Fluke 87 or an Air Fuel Ratio Monitor that is sold by many companies such as K&N, Holley, Edelbrock, TWM Induction, (Jegs and Summit Racing catalogs sell these.) etc… These manufactures sell their air fuel monitors for approx. $120-$170. You can build the same thing for approx. $15-$20 through your local Radio Shack, Norvac, or other Electronics store.

If you use a Digital Multi Meter, you have to use a high quality Digital Multi Meter such as the Fluke 87 due to the high impedance.

The O-2 sensor will respond to changes in air fuel ratio INSTANTANEOUSLY! If you have one cylinder miss fire on just one revolution of the crank, the O-2 sensor will detect it and your Multi Meter or Air Fuel Ratio monitor will show you a spike.

Below is a list of the Voltages referenced to Air Fuel ratio. 

 

Volts A/F ratio

.1 17:1
.2 16.5
.3 16:1
.4 15.4
.5 14.9
.6 14.4
.7 13.8
.8 13.2
.9 12.7
.985 12.1

If this next section is a little confusing, ignore it for now. Once you become familiar with using the O-2 sensor for tuning and diagnosing driveability problems, then this section will make more sense………..

Don’t think of the O2 sensors indications as rich or lean. Think of them as "all oxygen used up" (around 900mv) and "some oxygen left over after combustion" (around 100mv).

This may seem picky but if you see a low voltage and assume the engine is running lean, you may be looking at incomplete combustion or a miss. More significant yet is a jagged waveform at the high (rich?) side which might look like a cylinder that is lean but could be ‘puff’ of EGR gas (not rich) from a faulty valve. (if you are running an EGR valve.)

Sensors vary but popular pattern is a variation from 0mv to 100mv at the low side to 800mv to 900mv at the high side with an average of 450mv. Oxygen sensors will never stay at the mid point. They will flip-flop from seeing no spare oxygen to seeing some oxygen remaining in exhaust stream. They are incredible devices: so rapid and sensitive that, with a good scope, you can identify vacuum leaks and bad injectors. They are a very underutilized diagnostic tool.

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