XJ.89.Limited

Title says it all guys, how do I go about testing my O2 sensor? I have reason to believe that it's making my mpg lower than it should be. I'm averaging 10mpg on 235s and stock lift with a light foot on the skinny pedal. My friend that has a 96 with 3in lift and 31s gets better mpg than I do. He gets about 17mpg. I understand that when the cold winter comes around it should drop a little, but I've held a consistant 13mpg since last january. Still not the greatest, but I kinda just dealt with it. I did a tune-up and it didn't make a difference and my dad kept telling me the O2 sensor was fine and it has always run like that. I still have the original O2 sensor from 89 in there, and I finally got him to agree with me. Now I can finally look into it without him giving me hell over it SO if there's a way to test it using a multimeter, I'd like to know. I'd rather not go out and buy one if I don't have to.

XJ.89.Limited

BUMP

tjwalker

Don't bother testing it; it's shot if it's the original.

They aren't that expensive and they can't be accurately tested with just a meter. Get a new one. They have a finite lifespan.

Bosch is the OEM for the 87-90 4.0, NTK would be fine too.

my89xj

Just get a new one, it fixed half my problems, its worth what you save in gas.

cruiser54

For the Renix years, 87-90, the O2 sensor has 3 wires, 2 black and 1 orange. The orange wire (largest gauge of the 3) is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall, and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immedietly after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

One of the black wires is a common ground for the heater power and O2 signal to the ECU, so a poor ground will give a voltage feedback from the heater power input, to the ECU causing poor mileage even with a good O2 sensor.

The third wire, also black is a voltage feed wire, 5 volts, from the ECU to the O2 sensor. The O2 sensor is an O2 concentration sensitive variable resistor. At optimal O2 concentration the 5 volt input feed to the O2 sensor drops to 2.45 volts due to losses across the O2 sensor to ground. That same wire if disconnected from the O2 sensor will read 5 volts constant to ground.

At idle that voltage should read 1-4 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 2 and 3 volts max, and is optimally running between 2.3 and 2.6 volts at 2000 rpm (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the ground and the 12-14 volts to the heater and the 5 volt feed from the ECU with power on and engine off. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth with the engine runing.

If the O2 sensor readings are not right, say they read 4 volts or 1 volt steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 volts on the orange wire, and 5 volts on one of the two black wires), and ensure the ground wire (power off) reads less than 1 ohm to the battery negative post.

A leaky exhaust system or leaky fuel injector(s), or bad compression, bad rings or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.

XJ.89.Limited

Thanks cruiser! I'll take a look at all those things, but I'm fairly certain my O2 sensor is just shot. Gonna check the plugs soon to see if it's running rich. Plugs, cap, and rotor are all under a year old. Wires could be replaced, but those seem to be fine. I'll check compression another time, but my dad will most likely alienate that possibility when I tell him. He's a little stubborn sometimes. He's just the one that knows how to do most things, so I ask him. I'm still building experience on my own.

freegdr

How about cts ever change that ? while your down there ...just saying

XJ.89.Limited

What do you mean by CTS?

freegdr

Coolant temp sensor .

XJ.89.Limited

I know of the one on the radiator, not one down by the O2 sensor.

freegdr

one on radiator is for electric fan .Driver side engine block under intake/exhaust next to knock sensor.

freegdr

Here....

XJ.89.Limited

Is it hard to get to? How would I go about testing it? That was one of my last thoughts since it doesn't run hot or anything. I can see how it would affect my mpg.

my89xj

Not to hard to get too, I changed mine in like 5 mins

XJ.89.Limited

It seems like it would be pretty easy to get to. I have small hands, so most hard jobs are fine for me, haha. I'd rather test it first if I can rather than change it out and waste money.