Greetings!

Ive been trying to fix unusually high fuel consumption on my 97 xj6 3.2L for a while now. Factory new 3.2L should be around 12 L/100km (20 mpg) according to internet resources, but its been 27 years since it left factory, so I wouldnt be surprised if im getting 15 L/100km (16 mpg). Mine does closer to 25 (9 mpg), but I cant tell exactly how much as I cannot figure out how to read the fuel consumption on the dash (just some head calculations based on what I filled it up with and distance averages). The car is only used for short trips in town, so I dont expect perfect fuel consumption, but 25 is way too high - it does that even on motorways.
I also started having to get high idle RPM (about 1.3-1.5k, it intermittently varies by 200RPM - a service OBD2 scanner reveled this fluctuates based on the "compressor clutch feedback" going from LOW to HIGH) a while ago which made me fail the CO tests (they only measure idle CO here), so the car is not on the road for now until I fix it. I decided to check for vacuum leaks and clean out my air intake line while I replaced the gasket on the idle air intake valve on the throttle body (which didnt fix the high idle RPM), but when I removed the throttle body from the intake manifold, I noticed that the intake manifold is filled with a fuel/oil mixture at about a 0.5 - 1 cm level (very small amounts of oil - mostly fuel - might be years of accumulation).
A mechanic friend I know suggested I use some injector fuel additive, as this might be caused by gunked up injectors just leaking fuel instead of spraying it, but I might also require to change out the injectors that are faulty.
I have read on these forums that a poorly positioned throttle position sensor might cause high idle as well, but I havent tried to tackle that yet as I didnt have my multimeter with me at the time to measure its voltage.
There are no fault lights on and I have not managed to read any codes besides a known immobilizer fault code that makes my jag randomly sound the alarm, but thats a separate issue I havent had the time to tackle yet (search results say changing the boot switch/light harneses as they rust and break over time), as just removing the horn fuses works for me for now not to make my neighbors want to strangle me.

So I have come to these forums to share this information in hopes someone might have been dealing with a similar issue in the past, and can point me in the right direction of what kind of repairs and part replacements I should be focusing on.

What I did:
replace air filter;
clean mass air flow sensor;
clean throttle body;
clean throttle idle air intake valve and replace gasket;
clean every joint of all of these systems;
check for vacuum leaks by spraying some brake cleaner around the engine bay while the engine is running.

What I plan to do:
clean/replace upstream O2 sensor;
regulate the throttle position sensor to 0.6V at idle;
put in some petrol injector cleaner additive in my tank and run the car for a couple of hours (as i cant drive it anywhere for now).

What else should I check on? I am not an auto mechanic, but I am an engineer by trade so I wouldnt have an issue with some more advanced repairs, if needed. I would like to avoid part replacements for now as they need to be shipped in, this takes a while and I want to drive my jag again asap but of course, its unavoidable sometimes

Another thing id like to add is that the engine takes a while to reach neutral temperature (about 20-30 minutes), and the radiator fans are always running. I read that the fans should always be running on these cars as its a Jaguar fix for fuse blowouts, but it still seems weird to me to leave the engine running cold for so long - which adds to the high fuel consumption. Supposedly that shouldnt be related as the flow of coolant is regulated by a thermostat. Should I look into a new thermostat as well?

Thanks in advance!

The upstream O2 sensors have nothing to do with engine regulation.





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Yeah, you should plan on replacing that thermostat. You should know that your car either needs the o-ring or a gasket. The o-ring may come with the thermostat, the gasket will definitely not and will have to be ordered separately. Others with your engine may already know this answer. Its a relatively easy job, but you will lose some coolant. You might also plan for some extra time cleaning up the mating surfaces...

The TPS can manually be adjusted, if that's the problem, and it very well could be. You will have to enlargen the mounting holes on the TPS, and then tighten it back in place just enough so you can move it by hand with some force. Setting it back to 0.6v may not fix your issue because over the last 27 years, your ECU has slowly adapted to a new setting as the car aged. You'll want to reach under there, with everything back in place (remember, the TPS isn't locked in place yet, its just loose enough to move by hand with a little force) and with the engine running, reach under there and adjust the TPS position just enough so that its idling at about 700-720rpm, warm....so be careful! The other way to do this is to find a shop with a PDU/WDS/or some other device that can Reset the TPS. Should be charged no more than 1 hour of labor by the dealer.

Can you pull the LTFT numbers using your OBD2 reader? That would help.

Replacing the oxygen sensors would be a good thing too, if you don't know when it was last done, they probably owe you nothing and you should have them changed. If you're only going to do two of the four, the DOWNSTREAM sensors are the ones that tell the ECU how to fuel the engine. The upstream sensors are there to check to make sure the downstreams are working.

Last thing I could recommend are new spark plugs. They are cheap and easy to replace. Shouldn't cost you more than $15 for all six and should take you 30 minutes from when you grab your tools, to when you put your tools back.

Ahh yes, the fuel pressure regulator. Those RARELY fail, BUT if you want to check, yank the vacuum hose off. If its wet, or smells of fuel, the regulator is toast. (Parker is always on top of his game)

Thanks, ill try to just swap them first and check if anything changes. Forgot there are 4 on this car.
Ill order a new thermostat right now then.
About TPS - thanks for the "when hot" bit, i would have definitely forgot about it when I would perform TPS adjustment.
Spark plugs dont seem like a bad idea - in my mind a bad spark plug could cause the fuel to never be ignited and some might spray back into the air intake manifold - does that seem right?
O2 sensors - I definitely do not know when they were replaced. Do not own this car for very long and i havent had that much time to tinker around it as I wished lately. Will check if something changes when I swap them around and clean them, and buy new ones afterwards.
I have bought 3 separate elm 327 scanners and none of them showed me anything (4th is in the mail right now). I only managed to see any real code outputs with a professional scanner I happened to have access to for an hour or so - and that scanner couldnt even communicate with the ECU or most of the other systems. I am starting to suspect that the ECU on my jag is aftermarket, as its mounted weirdly (this always bugged me) ill check around and see when I get to see the car next.

So thats what it is. I'll try this as well, thanks!

All in all im surprised how fast I am getting responses and how logical they seem so far - did not expect this on a 25+ year old cars enthusiast forum. Thank You!
My car is not with me at the moment, as its getting some paint work, but ill bundle all of these suggestions up and try everything after a week or so. So please, keep them coming, nice people of Jag forums

2 options came up with the thermostat, might they both be interchangable? Also, both seem to have gaskets coming with them. Part number seems to be AJ86484

Okay, as I said, not a mechanic. As I understand, the "upstream" sensors measure the amount of O2 left over, which tells if the mixture is lean/rich, and the ''downstream'' sensors check if the catalytic converter works. Is my terminology just twisted?

No. The AJ16 is weird.

If you change the thermostat keep in mind that it does not take a large O - ring even if it comes with one , bad , cracks the thermostat housing as you clamp down

The O2 2 sensors can be swapped as all 4 are exactly alike

The O2 sensors are not used in engine regulation until the engine coolant sensor sees 88 C

So you can have either the theromosat bypassing or the sensor failed

The ECU looks at the 2 wire temp sensor and not the 1 wire temp sensor that only feeds the instrument cluster ( and not very accurate )

The 2 wire engine coolant temp sensor will read about 225 ohms resistance at 86 F asthe ngine warms up

See page 78

jagrepair.com/images/Training Guides/801S - 2000.pdf

At engine warm up it will read 225 as the engine closed loop regulation , proves thermostat

The 2 wire temp sensor is cheap and don't last forever like the fuel pressure regulator

Both are a generic part and cheap at the local auto parts store

Codes ? see page 121

I think O2 sensors are read when the car goes into closed loop when coolant is read at 160F. At 88C, the thermostats are fully opened!

By the way, there are no aftermarket ECUs for these cars.

You're likely to get better prices and more accurate selection of parts on RockAuto.

Thanks for the detailed explanation!
I will add these suggestions to the list of things to check.

As for the tendency for the responses gravitating towards assuming I am in NA - I am based in Europe, so I have access to UK based parts shops more reliably. The OEM parts (if available) dont really cost too much more than matching generic parts.
A private message I received suggested the 3.2L displacement engines (which as I understand were only available in Europe) only have two O2 sensors and no EGR, so I might as well just order new ones just for sanity sake - after I check if that is really the case.

The O2 sensors are the uncommon Titania based sensors

If you remove them the wires ( pigtails ) are brittle galvanized coated , so care not to twist them

If you can read a number off of them , we may be able to cross reference them to be the correct ones

The connectors to the O2 sensors can be crossed as they are identical connectors

The connectors on the car side will have 3 wires of the same color but the 4th wire being different , this determines position

I had some green corrosion in my rear O2 sensor connectors

The is 1 fuse for all 4 sensors to heat them to be able to read

Fuse # 14 / 10 amp right engine bay fuse box , the # 1 fuse is closest to the fuse box terminal post

Pic coming

PSA - be careful with oversidzed orings when changing thermostat
i was just changing the thermostat and decided to use the oring that came with the thermostat, and while tightening the bottom screw where there is poor visibility, i didnt notice how the cover is bending at the bolt fitting place, and the it broke right off. Did not even have any resistance, it just broke off. Postponing most tests to when a new cover arrives in mail, as i dare not to run the engine right now

could be that i just positioned it poorly in a hurry, or it was under a lot of pressure before as the previous owner didnt use any gaskets at all, just sealant, but then again the metal used here seems to be some brittle alloy

while were at it, what are these 4 random unconnected harneses around my abs motor (i have a code saying my abs motor is dead btw after 10km/h, was just thinking of replacing it)
black and blue are attached to the same harness the abs control module is, and red and the optical cable looking wire come from what i think is the engine controller, as the other ends are connected to the injection part of the engine
Edit - also, this is the weird looking ecu i was reffering to, but i have now googled what it is and its a LPG control module and i guess this car used to have a LPG system. I wonder if that means someone has tinkered with fuel/air charts or whatever is the right term. Its a diego g3 module.
Bear with me, im new to all this, just doing research on the go.

Thanks for this picture, I just printed it out. I pulled my codes yesterday and came up with P1191, P1192, P1185 and P1186. Does that sound like a blown fuse to the heater. I'll check it today when it warms up a little. The Open and Shorted codes have me a little confused.