Switch To O2/Unleaded ????

I discovered just yesterday after having my XJ in the family for more than 10 years that I didnt have an O2 sensor -Plugged with a bolt-

It appears that cars assembeled here Egypt were setup to use UNLEADED fuel. But I started using unleaded fuel several years ago .

What are the advantages or disadvantages of switching to an O2 sensor system ? & how would I do that ?

If you have the rest of the electronics for it (in the engine controller,) the HEGO sensor provides a feedback loop to trim fuel delivery, which can net you a slight gain in fuel economy, possibly power output as well.

However, it will do you no good whatever without the control electronics for it. What year is your rig? If it's 1991 or later, do you have a CEL/MIL (Check Engine Lamp/Maintenance Indicated Lamp) lit? If you have the CEL/MIL, see if you can "pull codes" and find out if you're getting a sensor failure code for the HEGO. If you're not, then you probably don't have the electronics to support the sensor, either (it's possible with export models,) and you're better off just leaving the block-off bung in place.

HEGO = Heated Exhaust Gas Oxygen (sensor)
CEL/MIL = Check Engine Lamp/Maintenance Indicated Lamp

I have the CEL & I always get 55 :spin1:
I have the plug in the downpipe

mine is a 94 HO Egyptian specs assembeled here, but I think I can get an ECU from a US spec XJ which PN to look for.

What are the possibilities for better MPG my avg is 14 & i top 15 thats with a 155k AW4 3.55 235/75R15 ?

Hm. I can understand the CEL, but OBD-I Code 55 is "Display Finished" - it's told you all it knows. Are you certain that's the only code you're getting? (I'm not doubting you - just naturally sceptical. Come with working on so many different things, and having to troubleshoot half of them by "remote control.")

I don't recall there being a huge difference between Egyptian Specs and US-Specs, but I'd have to dig up that pdf of the OEM parts catalogue and see what I can see anyhow. If you don't hear from me in a day or so, could you email me a reminder? 5-90 AT naxja DOT org is my "Jeep Enquiries" address.

Back in 94 we didnt have unleaded which explains it 3 years later it was available.

There was a guy from venezuela on JU that had the same issue he tried to install a HEGO but his consumption dropped from 10 to 9 mpg. I think the ECU for the unleaded versions is different .
I drive mine 75 % of the time at 60 mph & I m getting 14s never saw 18 or 19 & I think my engine is still in shape.

I'm not an expert on XJ ECMs, but logic tells me it probably doesn't operate much different than a GM ECM of the same era.
I have had experience with my '94 GM ECM, regarding being set-up to run leaded vs unleaded. With my GM ECM, the PROM contains the fuel and spark timing tables that are used to control the engine. If the PROM is for leaded gas ('94 export to Middle East, and California off-road cert (commercial vehicles)), it ignores the O2 input, and only uses the internal tables.
I know this because the first PROM I had installed in my Chevy-powered XJ, was for leaded fuel. I didn't know that was the case, and spent a serious amount of trouble-shooting time trying to figure out why the system would not go into closed loop. The engine ran fine, and did not set any codes. After changing to a correct PROM, nothing appeared to change, as far as engine operation was concerned, but it would now go into closed-loop, and give me a code if I disconnected the O2 sensor. It would also now pass Calif SMOG.:sunshine:
I'd expect the XJ ECM's to be similar to the GM. If your XJ was expecting an O2 input (ECM configured for non-leaded), and none was there, you should get a code. If your ECM is configured for leaded gas, and you run unleaded, I'd expect, at minimum, poor gas mileage.
The fuel/spark tables will not be optimised for unleaded, and I'd expect the knock sensor is going to sense more "pinging" and keep timing retarded, further exasperating mileage.
You need to determine if in fact your ECM is intended to run without an O2 sensor (open-loop, on leaded gas). If it is, and you want to run an O2, you will most likely need to change ECM's (not sure if you can change PROM's, like you can in the GM ECM).

What are the possible improvements in MPG I might be seeing ?

Pharaoh XJ said:

What are the possible improvements in MPG I might be seeing ?

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That's the difficulty. If you're designed and calibrated for a HEGO, you should see a slight improvement (provided you stick to unleaded after the addition. The tetraethyl lead used in "leaded" fuel tends to clog the pores in the HEGO cell, rendering it useless in rather short order.)

xjbubba - while the outward operation should be similar to that for a GM ECU, the internal operations are quite different. That's part of why you can easily get a GM ECU reprogrammed, while it's more difficult for a ChryCo...

And, I think most of the Middle East-bound vehicles (Araby & North Africa) may have been calibrated for fuel using TEL or some similar organometallic, which does require a different calibration on the ECU.

Pharoah - when you say "started using unleaded years ago," are you also implying that regular (leaded) fuel was originally used, and you changed during your (family's) ownership of the vehicle? Just gathering information...

5-90 said:

Pharoah - when you say "started using unleaded years ago," are you also implying that regular (leaded) fuel was originally used, and you changed during your (family's) ownership of the vehicle? Just gathering information...

Click to expand...

There is no more leaded fuel available here in Egypt we only get Unleaded fuel here & 92 octane so its my only option, I cant really recall when it started being available here but i think 1998 or 1999.

So I m sure that i ll be running unleaded only thats why I want to take advantage of that.

I think TEL has been outlawed pretty much everywhere by now, so I wasn't really worried about it being run again. Mainly, I'm just wondering if you have electronic support for the HEGO sensor in the first place (otherwise, it will just be a more expensive bung plug.)

And just because you have the electrical plug doesn't mean you have the electronics, either. Cars are funny that way - it's easier to make a "standard" wiring harness and hardwire the features into the electronics (or not) than it is to make several variations on the wiring harness itself.