O2 Sensor readings
 

Hi,

Recently I installed an O2 (2 wire) sensor on my 78 SC engine. It has an after market EFI and computer and at this point I have not connected the o2 sensor to the computer. Instead I am using O2 guage. When the car is being driven the guage shows the correct readings.....however when it is stopped and I rev the engine the guage does not move, constantly reading rich, until I take my foot off the throttle, then it goess stoich.

Since the O2 sensor only measures fuel/air why would it need load on the engine. Is this related to back pressures- which is there at all times?

Cheers

Mark.....

You mentioned it is two wires which means it is un-heated.

For the O2 sensor to work properly it must reach a certain temperture. At idle or no load there isn't enough heat to make it work. Thus the reason you are not getting anything from it at idle.

The solution is to use a 4-wire sensor. These type of sensors have an internal heating element which keeps the sensor hot even at idle or low loads.

Note however that the 1, 2 or 4 wire O2 sensors (narrow band O2 sensors) only tell you if your running rich or lean not much else (above or below 14.7 A/F). Don't try and get an A/F reading from them, you never will get anything accurate.

If you want to get A/F readings from an O2 sensor you need a wide band O2 sensor which requires a special controller to operate them.

tbitz has a good point, another thing to look at is that if you have not hooked up the O2 sensor to the EFI, then you are running open loop all the time. This means that the O2 reading will not ocsillate like usual.

You say it is reading rich at idle.

What voltage are you seeing? Rich is above 0.5 volts and lean is below 0.5 volts. If the O2 sensor is not hot enough you would see little to no voltage out of the sensor (showing lean).

Also note the drive streagth of the O2 sensor is very small, which means you need a volt meter with a fairly high input impedance. Usually a digital voltmeter is good. The older analog meters with the indicator needle may not have a high enough input impedance.

I am using an A/F meter (led type) and have not tested the voltage. If it is not getting hot enough at ide or with no load then why does it change from rich to stoich when the revs are coming down after a blip of the throttle. Is it bacause there is enough heat left in the exhaust?

When the power is turned on, cold engine, the reading is rich not lean (0v cold) it stays that way until it warms up. Actually with nothing connected it reads rich so it may be that its normal state to read rich until a voltage is applied.

At stoich does an o2 sensor have 0v.

The meter that I am using can be used for a 3 wire sensor. ie power earth and signal. I have connected only signal and earth. It seams to work in that I know the engine is running rich and it moves to lean at times when it starts to backfire and pop, ie lean.

With a narrow band sensor you mentioned that it only gives rich or lean......I can see stoich readings under hard accelaration and at overrun. Am I being optamistic at seeing a steady stoich reading whilst cruising? I have a handset for the computer so I can make changes on the fly.

OK should I invest in a 3 wire wide bandwidth o2 sensor.

Thanks for your help

Mark.....

I have not hooked the o2 sensor to the EFI because the o2 sensor input does not seam to work. It constantly reads 17:1 wheather it is grounded of fed .05 volts. This could be because when the engine was installed a 12volt wire has been attached to the senros connector! Time for a new computer!

Some basic info on O2 sensors here :
http://home.planet.nl/~Jennit/Technical/O2_Sensor.htm


Here is a typical response of a NB O2 sensor:

http://www.pelicanparts.com/techarti...nitor/FIG1.JPG

http://www.pelicanparts.com/techarti...nitor/FIG2.JPG

Wide band O2 sensor is 5 wires and requires a controller. This setup is not cheap. For more info check out:

http://www.techedge.com.au/

http://www.diy-wb.com/