XJ95 oxygen sensor and potential upstream problems
 

Can the 4.0L be driven while a CHECK ENGINE lamp indicating oxygen sensor has illuminated? I have driven it about 300km already with the lamp switching off between starts. It is not currently illuminating. I have read my 95xj has only one sensor whereas the 96 + has two.

Appears to be running fine and not missing a beat, but is it a bad sensor or indicative of an upstream problem?

I have noticed that for a while, my temp gauge is not going up into the 90s (it used to be at least 80 even downhill), except on hot days and in traffic or stopped at lights, tripping the fan at about 105 degrees (normal behaviour) Thermostat was last changed about 8 years ago (genuine part) Could it be that the fuel (ULP 95) is not burning properly? I am not noticing bad fuel economy either.

Should I leave things as they are, or at least change the O2 sensor and see if the check engine warning indicates o2 sensor again? If the warning still appears after a new sensor, does this mean a new thermostat?

I should also mention that a hairline crack is developing in the air inlet manifold (28 yo). Could this be affecting the temp gauge and how the fuel burns? The mechanic had a look at the manifold but did not flag it as a problem - yet.

Yes, absolutely afaic...and Chat GPT apparently agrees with me :)

The mass air flow sensor (MAF) is one of the most important engine management sensors in a Jeep Cherokee. It measures the density of the air entering the intake manifold and passes this information onto your Cherokee’s ECU. The ECU uses this information along with other sensor data to control the amount of fuel that enters the combustion chamber


I would ask the mechanic what he thinks happens when unmetered air that is not detected by the PCM enters the combustion process

If the crack is big enough, the engine may not be able to enrichen enough

If that was the case, you would expect poor fuel consumption

You could try sealing the crack with a blob of silicone.
O2 sensors work in a tough environment, the fuel we get now doesnt help either.
Injector cleaner may help.

Simple test for the manifold 'crack'....with the engine cold and running...very important the engine has just been started and is cold(a hot exhaust may ignite the fuel)....use a spray bottle with fuel to spray around the 'crack'....if engine increases in revs...'crack' is open to atmosphere and needs attention...no increase in engine revs....'crack' is not significant and can be discarded as an issue.

"Start You Bastard" is Ether and works great for that available at Supercheap and most auto parts stores

Even safer is use brake cleaner as a leak detector.
If the ECU says the 02 sensors is bad then its strategy is to operate open loop and the fuel delivery follows an internal map inside the ECU. Then it’s delivery follows the map schedule is based on rpm, mass flow, temp, and other available sensor inputs but it is no longer fine tuned by the 02 feedback, so the performance and economy will not be far off what the engine demands.

Thanks for all the good advice.

I have done a couple of 300km round trips in it since I first reported the CHECK ENGINE o2 sensor problem. The car has not missed a beat, and I have not noticed any excess fuel use. I also haven't seen any sign of a coolant leak in the engine bay or on the ground.

The CHECK engine comes up every couple of starts for the same. Is it OK to keep driving it? The car idles and runs perfectly. Classic Boy, I think you may have answered that one.

I would like to know where I might (eventually) be able to find an XJ 1995 compatible air intake manifold. Nothing on the web. Can these things be repaired? I have noticed that the fuel injector plugs are starting to stain the manifold. So maybe they are also part of the problem. Everything is approaching 30yo (400K km, which when I think about it, is unbelievable. Original owner.