Oxygen sensors help the computer make corrections in the car’s air/fuel ratio by measuring the oxygen content in the exhaust. Oxygen in the exhaust is a good measure of the efficiency of the combustion.
The computer then adjusts the air/fuel accordingly. In CIS cars – it changes the duty cycle (On-Off) of the frequency valve, which is the buzzy thing on the back of the fuel distributor on top of the air box. In CIS-E cars, it adjusts current to the DPR (Differential Pressure Regulator) to change the pressure in real-time.
Oxygen sensors go bad for a few different reasons. Leaded fuel will kill them. RTV silicone hurts them, and lots of soot will clog them. The wire can also become frayed and broken and not carry the proper amount of voltage. On newer cars with heated sensors, the heater may break, causing the warm-up time to take longer.
Testing the Oxygen sensor
Using a voltmeter, and the output voltage from the oxygen sensor on a warmed up car should be between 100 and 900mv and fluctuating (after elevating engine speed to 2000 rpm for a few minutes), and 450mV is stoichiometric (perfect for complete combustion). Lower would be lean, higher would be rich, which is always safer. No voltage, and the puppy’s dead or not connected. (Big thanks to Cathy Boyko for this test!)
Lumpy, surging idle
The reason that the lumpy idle happens is that the oxygen sensor’s reaction time gets really slow as it gets older. Here’s a good way to trouble shoot this. Warm up your car until it starts doing it’s lumpy thing. Lumping? good. Now, push your full throttle switch on the throttle body, and hold it in and see how the idle changes. Lumpyness go away? Then it is most definitely your oxygen sensor that is bad.
A universal one wire oxygen sensor is cheap – and easy to get. for an 8v, you can get one for between $20 and $30 from an autoparts chain. 16v cars have a heated oxygen sensor that costs more and has more wires on it.
Then go get an oxygen sensor socket, usually about $10 at a parts store. It’s a 22mm socket, and it’s actually useful for more than just this. It works on the strut nuts on the front, and on the windshield wiper holes through the cowl.
For the 16v, it is easier to use a 22mm open end wrench, the sensor is mounted on the cat, pointing slightly up underneath the car.
Wideband Oxygen Sensors
Wideband oxygen sensors can measure the air/fuel ratio way more accurately than a narrowband one can. People with standalone fueling systems like Megasquirt or SDS can benefit a lot from this input. A narrowband sensor only has three states. It tells the engine when the mixture is at stoichiometric (14.7:1), it can tell if it’s too high, or too low. But it does not tell how much. The wideband has a linear scale that corresponds to all different ratios of air/fuel. Then the ECU can more easily adjust the mixture exactly.
The Innovate LC-1 or LM-1 use a Bosch wideband sensor that was used on Mk4 1.8T motors. It plugs right in to the LC-1. It’s less expensive to buy the LC-1 from a tuner, and then buy the sensor from a car parts store. Just ask for a 2002 Beetle turbo front oxygen sensor.