All modern production cars have an O2 sensor in the exhaust.
Basically an O2 sensor is a battery that produces a voltage when exposed to oxygen.
Oxygen is present in the exhaust system of any internal combustion engine and the amount of oxygen present is a factor of the fuel/air ratio being burnt.
Everyone knows that the correct Stoichiometric ratio is 14.1 parts air to 1 part fuel and it is this ratio that you are adjusting when twiddling the knurled knob on the back of your injection pump.
Now, the o2 sensor in your modern car is known as a "narrow band" sensor because the air fuel ratio is carefully metered by very modern equipment and so deviations from stoichiometric perfection are small and therefore the voltage variations produced by the sensor will be correspondingly small. (About 1v range). So, when your modern EFI is working correctly the voltage that is generated by the sensor will be fluctuating between +0.5V and -O.5V. This fluctuation is a result of the ECU constantly reading the sensor voltage and making corrections to the fuel volume injected in order to keep the voltage at an average of 0V.
This narrow range is fine for modern EFI cars, but is no good for tuning application where a wider range of mixtures might be expected or desired so there is a "wide band" O2 sensor which operates over a range of about 6V that can be used in more "abnormal" situations like those found on our cars.
Since our cars were not fitted with O2 sensors when new we must install a fitting so that we can insert one. These threaded fittings are commonly available from most tune-up suppliers along with a blanking plug to close the hole up afterwards.
There is a fair amount of information on this site about how to fine-tune your injection pump and this information used in conjunction with a wide band O2 sensor will yield excellent results if patience is combined with a methodical approach.
So, you cut a hole in your exhaust downpipe and weld in the threaded insert. (There are other ways but weld-in is best). You then install a wide band O2 sensor and run a cable from it into the inside of your car. (Through the window is fine as you'll be removing it later). You connect this cable to a display which you temporarily affix inside the car.
Using this display you will drive and tune, drive and tune, drive and tune until you get as close to 14.1 on the display as possible under normal warm conditions and anywhere between 11 and 14.1 when cold or under full load. (lower is rich, higher is lean). The only time lean is acceptable is on over-run. In reality you'll be happy with 14.5 or 13.7. Naturally, you will then want to check that you are getting enriched mixture when cold but you must establish the correct setting when warm first.
Once you are satisfied you simply remove the display, cable and sensor, install a blanking plug into the threaded insert and enjoy!
