Questions and Answers

Isuzu O2 Sensor Problem? Q. I just purchased a 1991 Isuzu Rodeo for my son for college and while I drove it for about 300 miles the O2 light came on, here are the specific's: 2.8 liter 4 cylinder Fuel Injected 2 wd 90,000 miles

When purchase the previous owner indicated that the check engine light had come on and he had to change the Master Control Unit. Prior to the O2 light coming on the engine seemed to run rough at idle but ran like a top on the open road, I saw no smoke at any time with acceleration. Once the O2 light came on, unless it was my imagination the car idled smoother and still ran like a top on the open road. I averaged about 17 MPG in town and about 24-26 MPG on the road.

I have an electronic back ground and have always worked on my own cars. I replaced the O2 sensor and the light is still on. I removed the MCU fuse in the driver's compartment to reset all codes but the light is still on. According to the Clinton book the sensor should be giving me a .4 to .5 DC volt reading but my reading (Using a digital meter) is more like .93 to .97VDC. When I disconnect a vacuum line the voltage drops to zero.

I'm assuming that the light is still on because the voltage is too high and the reason it's too high is the engine is running too rich so do you have any idea's as to what I can do next?? And by the way do you know where I can find the connector for reading failure codes? I'm having trouble finding it under the dash to the left of the steering wheel.

You have a great site!
Rich

A. The oxygen sensor provides a signal that is factored into ECM air/fuel ratio calculations and adjustments. The sensor generates a voltage based on the amount of oxygen in the exhaust gas as compared to outside air, thereby determining the rich or lean condition of the incoming air/fuel mixture. As exhaust gas oxygen content rises (lean mixture) voltage drops. Conversely, as oxygen content falls (rich mixture) voltage rises. Voltage generated by the sensor varies between approximately 0.1V and 1.0V. So a normally operating O2 sensor will be constantly ranging from 1.0 volts to 0.1 volts in open loop and around .5 volts in closed loop.

Now what you need to be aware of is that the O2 sensor itself may not be the problem but a symptom of another problem. If the fuel mixture is too rich, it may be beyond the ability of the O2 sensor to compensate. If this is the case you will get an O2 sensor failure code. By unplugging the vacuum line and the O2 sensor voltage drops to 0.1 volts, that indicates the O2 sensor is working and the problem is elsewhere.

To put the ECU in diagnostic mode, you plug the DLC test leads together and the DTC's will display.

I would pull the DTC's from the computer and see what other codes are stored. If there are none I would probably look at the Fuel Pressure regulator and make sure the fuel pressure is within specifications. If it is too high, too much fuel will enter the engine causing a rich mixture.

Fuel Pressure specification

No vacuum to regulator	294 kPa (42 psi) at idle
With vacuum to regulator	245 kPa (35 psi) at idle

Additional Information provided courtesy of ALLDATA