Before I begin, this is a bit of a long post. I am still learning about this vehicle platform, and look forward to the feedback (harsh or otherwise) on what I should tackle next, and mistakes I might have made along the way.
This does take place over several years, so some details I simply do not remember super clearly.
*And before you direct me to another high-idle X300 thread, I had spent more time reading them over and over again at 2am than I have working on the car.*

Chasing the high idle issue on a 1995 Jaguar XJ6 X300
Since 2022, our family project car—a 1995 Jaguar XJ6 X300—has been plagued by a persistent high idle issue.
And I believe it is all my fault.

During maintenance of the ******* hose, the coolant temperature sensor was unplugged to avoid damage. However, starting the car with this sensor disconnected may have affected the ECU's baseline settings, leading to persistent high idle.

Realizing the mistake, the car was turned off, and the sensor plugged in.
The high idle has persisted ever since.

Since that fateful day, we have tried (roughly in order of what and when it happened)


Tested the coolant temperature sensor, going throughout the entire temperature range from ice cold to boiling.
Results: Nominal

Throttle plate grub screw was backed out in its entirety. This resulted in the Throttle Position Sensor reading too low - but we will get to that later.
Results: No change

ECU hard reset. Leaving the battery unplugged and the positive and negative leads touching for more than 10 minutes.
Results: No change

MAF was taken from my dads 1996 Vanden Plas and plugged in.
Results: No change and original MAF was returned

Taking a break from the idle issue, sometimes the transmission started acting funny after some spirited driving. I actually don’t remember if this was before or after the idle issues. The transmission control module was switched out for one from a 1996 XJ6L from a buddy’s doner car he had picked up. This, and in combination with several fluid flushes, gear selector slider adjustment meant I had to leave the car before going back to school across the country. It would not downshift unless at a complete stop.

The car bounced back and forth between my dad and brother as the primary care-takers, and has ended up back in my hands. In that time, the transmission issues have mostly been fixed, along with the high pressure lines for cooling the fluid down that go to the lower part of the radiator, along with the downshifting issue.

Now that it's my issue again, and my only car; it simply has to work.
My first thing to check was any intake leaks post MAF. There were some suspect irregularities between the bellows-esque part that attaches into the throttle body, but I RTV'ed that on to the pipe which came before and very thoroughly inspected for cracking and other leaks.
Results: No change

Next was the IACV or ISCV, whichever you prefer. This is the plunger that controls the IDLE SPEED. This seems to be a commonly used product since Jaguar had been bought by Ford. My options were getting one for ten dollars or one hundred and fifty bucks. I got the cheap one. When I got everything else buttoned up, and performed an ECU reset, the first drive of the car was magical. The like-new spot on idle. As soon as me and the wife got some food and restarted the car, the idle was higher than it had previously been prior to the IACV fix.

I almost lost my temper, as the parking lot was busy, and needing to stamp on the brakes to stop the car from running away past second gear without my foot on the gas. So so close to success!
Results: Change, then after hot-start, no change

At this point, I wanted to revisit the coolant temperature sensor, and eventually get to the TPS. That being said, I had just gotten a job with a thirty five minute highway drive away, and riding my bike with the temperature dropping over two hours in the morning and again during the night isn’t feasible.

But in that time, I have pulled the ECU, cleaned the pins and connectors, and checked the voltage and resistance from the coolant temperature sensor both at the ECU pins and at the sensor. Good news is that the connection is solid. While pulling out and cleaning the ECU, I found, and removed, the rats nest of an aftermarket security system that was spliced into the factory harness in several places. The door locks don't work anyway, so at this point it needs to be done. Another item to the list to fix!
Results: No change.

For context, the MPG I’m getting was fluctuating from thirteen mpg in town to seventeen on average while commuting to work, with 22-25 on the highway according to the onboard computer.

The car always smells like it's running a little rich - the wife agrees with this observation.

Time for the Throttle Position Sensor. I put this off because I was tired of pulling out the intake tubes, but I have figured out a fast and efficient way to do it by now, and working on more critical systems of the car does not seem as daunting anymore. Throttle body off, and multi-meter out.

– It is worth mentioning that I did clean the Throttle Body and throttle plate when installing the IACV. –

This circles back to the throttle plate adjustment/grub screw that was fiddled with at the beginning of this process of diagnostics. With the screw at its current position, the throttle plate was sticking ever so slightly when closed. This, however, never was an issue due to not having traction control on this base model, no sunroof superleggera X300.

I removed the Throttle Position Sensor after measuring its resistance without power which was low - around .55 Ohms (This tracks right as that damn grub screw was tampered with…) and gently cleaned and reassembled. With the screw adjusted roughly at .62 ohms, it was time for re-installation. Now at this point (as of writing, this happened yesterday) I had thought ahead and tested the voltage and resistance of the TPS before bolting it all back in place. Of course, from its unpowered state to its powered state it was higher than .62, falling around .75… with some careful adjustments and several more measures with the multi-meter, it was hovering around .620 – .615. Fantastic. Bolted back up, reset ECU. And…….

Same high idle of around 1500, and 1200 warmed up.
Results: No change, but my expensive TPS is still good!

Now here is what happened today on the day of writing. After another ECU drain and reset overnight, I turned to the EGR. Yanked the EGR valve off, traced out the gasket on the back of an Altoids case, cut it out with my Leatherman leaving the channels blocked off in case the EGR is stuck open. Bolted it back up, started the car ~1500rpm~ warmed it up ~1250rpm~ and went for a drive around the block. All the characteristics are the same as before, so maybe the EGR wasn’t working anyway.
Results: No change

Other things I have tried is clamping off the valve train breather hose while the car is running
Results: No change

The observable facts are:

*Idle is usually 1200-1500rpm on a cold start. 1500-1700rpm on a hot start - This is slightly randomized though.

*ECU reads idles 250-300 rpm lower than they really are.

*Idle WAS fixed after IACV replacement

*Fuel economy is Ok… I do drive very conservatively and gently (US 17mpg combined, mostly highway under or about 70MPH)

*There are no intake leaks post MAF

*All sensors seem to be functioning
Questions for the universe:

Would this mean that the 02 sensors are seeing too much oxygen and enriching the AFR’s?

Would this mean the IACV is getting jammed open and/or there is too much resistance between the ECU and the IACV for it to move back to its proper plunger location - making this a wiring issue?

This is my favorite car, and I plan on driving it until... well, forever. We have three X300's in the family and this was the first and most neglected that got us into Jaguars (X300's in particular).
Much much more has happened to this car than mentioned here, this is just addressing the idle issue.
Any feedback or comments are appreciated!

Sources and Threads used:
https://www.jaguarforums.com/forum/x...stment-248513/
https://www.jaguarforums.com/forum/x...e-cured-80285/
Jaguar XJ6; Jaguar X300; 1995 Jaguar XJ6 3.2 Sport
And many more...

 

No harsh response but here we go as a professional respance to your becoming a Jaguar Man ( I'm not there yet as contributions from others )

The engine regulation will not go into closed loop ( effecient ) until the ECT sensor sees 88 C or 275 ohms or below

Cold engine around 1800 ohms

Only when the engine gets into closed loop will the O2 sensors be used for fuel trim / mixture so we have to determain if in closed loop

The TPS sensor can be a can of worms int it's values at the idle stop , do not adjust the TPS or the throttle idle stop screw just yet

Have you cleaned the throttle body of internal guck yet ?

Removed the exhaust manifold cover and inspect the casting for cracks

See your AJ16 engine starting on page 51

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

Your TPS should read 0.60 volts DC + or - 0.02 on paper at the idle stop on middle connector wire Green / Yellow as it goes over the fuel rail

My TPS connector lock bar was missing as I received M' Lady Penelope , so bad connection

There is a TSB R493 in 3 parts on the throttle cable return spring upgrade and there is a work around using the original spring

05.1-26 amend4 (R493)

Does the idle issue resolve by manually by hand placing the throttle butterfly on idle mechanical stop ?

Ask Questions

There are some free things you can do with your O2 sensors

 

Did you have the O2 sensor plugs undone at the back end of the fuel rail at any time? They are identical and if they got swapped the engine would probably never be happy without a new O2 Orientation done with the rare and mystical Jaguar PDU. Not to say that high idle would necessarily be the result.

Your dad probably already asked you this.

Once you solve this problem the car should be 100% reliable. Good work so far.

 

The easy way of checking this if you have a code reader is unplugging one of the sensors, follow the wire down to the downpipe to see if its plugged into bank 1 or 2, then checking to see if the corresponding bank shows up on the error code after starting the car.

 

probably not going to offer you much help but i went through exactly the same stuff....
only thing that fixed my high idle was getting someone with a WDS/PDU to do a TPS reset.
it worked.

other issue i had recently was rich running even to the point of stalling.
went through everything, ecu swap and all, turns out it was likely leaky fuel injectors.

only thing i can suggest is get someone with a PDU to do a TPS reset and a O2 calibration.
i would have to think this will fix your problems and nothing else will no matter what you do.

 

There is a IAC valve exercise / test that that Vee has I never grabbed and put in my computer and maybe you can put that to rest

There should be a gasket between the IAC valve and TB

The mount bolts can snap off as Loctited

 

Being in US you will have 4 O2 sensors that can be swapped around ( same exact connector and pigtail )

The pigtail wires are very brittle so no twisting , galvanized wires , 17 mm sensor hex for removal

There should be a 12 mm crush washer for the O2 sensors , your spark plugs are 14 mm , for a new sensor Bosch # 13789 connector ready equivalent , there is a Walker #

only the aft are used for engine regulation , the front is used for catalytic efficiency verification and gives a distinct code

There is a IAC valve exercise / test that Vee has

The IAC mount bolts can snap off , loctited , 5.5 mm

There should be a gasket between the IAC valve body and TB , but you've had this car for a long time

The O2 sensor connectors at the rear can get crossed on connector hookup

there is a wire color to follow

You have some connector , grounds cleaning you can do for the O2 sensors and 1 fuse for all 4 heating elements inside the sensors to read correctly , you can put a common meter on the sensors heating elements at the connector to verify that 1/2 of sensor good

External ground wire / strap on the ECU " case " mount bolt ?

# 14 / 10 amp red right engine bay fuse box for heaters

Question on if the O2 sensors are being biased as reading a high unconsumed O2 content :

Exhaust manifold leaks ( casting cracks , donut gaskets loose ) and whole engine vacuum line leaks

The vacuum leaks have a behavior of as you bring the engine speed up the supply of vacuum overcomes the amount of vacuum leak so fuel trim returns back to target 0.0 %

Have you read for any engine CEL codes ?

 

Parker7! Its an honor. I'll try and answer to the best I can and clarify details.

*Throttle body is quite clean at this point.
*TPS was previously fussed with unknowingly with the set screw, so I tried to set it back to a neutral state in this most recent mechanical dive -- .615-.620 -- as measured from the middle connector wire.
*Exhaust manifold has had its cracks repaired (! thank you cafe flyer), but I'll double check the torque of the nuts.
*My connector lock bar is missing as well. The pins in the connector seem to make contact but maybe that's part of the issue - but it still seems to read the voltages.
Throttle cable return springs work great. There is no change in idle RPM whatsoever when I try and push the throttle plate back into position - before and after loosening the throttle cable a slight amount.
*As to the IAC valve, the new valve came with an O-ring. I tried all combinations with the original gasket, O-ring, and the little metal ring that comes out that the O-ring fits into. The last combo with the O-ring minus the OEM gasket was when I had the magical moment. The bolts were not too bad to remove. I was very careful.
*We did clean the O2 sensors back in 2022, taking great care to not fudge the wires when screwing and unscrewing them. I do not believe I removed the aft sensors to clean them very well, but they looked lovely when I removed them. *for clarity, this is post idle issue as well*
*ECU ground looked good, and being a desert car there is so little corrosion anywhere its wonderful. I will remove and reinstall the ground bolt though just to be sure.
*I will check the fuse you suggested tomorrow before work!
*There are no check engine lights, but my scanner is not very advanced. I have been using this: https://a.co/d/dFclVNM which can only seem to read RPM, speed, and voltages - does not seem to get the rest of the Jaguar memo.

Oh boy. I am not looking forward to fighting the O2 sensors again.

I spent tonight with the old and new coolant temp. sensor building a data set.
There seems to be some variations in the middle, but nothing too off at the extremes.

... Time to get some sleep!

 

Oh man, I'll have to hunt for a PDU... Or someone who has one. Detroit metro area is not exactly *classic* jaguar territory that I have observed!

 

The 02 sensors have been cleaned but I don't think they were unplugged. This was back in 2022 when the issues first arose so I don't think they have been. Could be wrong though! Will have to bug cafe flyer to see if he remembers.
The car still takes several hour road trips! Just frustrating coming into gas station lots with a high revving engine Can't wait to get this one sorted out, and find out what to look for in the future.

 

@WaldenFix O2 sensors were cleaned, all four of them, one at a time, and were never unplugged. I was VERY careful with the twisting and untwisting of the wires... vague memories of two extra long ratchet extensions, a crows foot 17mm crescent, and the smell of PB Blaster on the threads to help loosen them. If I was good, I remember putting them back on with the copper grease on the threads (I did!)

Exhaust manifolds were replaced almost exactly three years ago - hopefully they have not cracked during that time!

 

1. No such thing as a hard reset on these cars.

2. Messing with the grub screw may have been a mistake. You will need a 0.002" feeler and reset that butterfly so that it has a 0.002" gap when "closed" or at idle.

3. You need a TPS reset. Only can be done with a device loaded up with Jag software. A Jag dealer should be able to do it. Shouldn't cost more than one hour of labor for this 5 minute effort. If you have british car shops, they should have the means to do this as well. Some larger shops could have the means too, but you should absolutely call ahead. You can also try to find a device online that can do it. The chinese cloned Mongooses and VCMs will work, but not for long, so if you buy one, get in, get out, and put that device in a drawer for emergencies only.

I've always suspected that the TPS can me adjusted manually, but I now think that is not feasible for whatever reason. Get the TPS reset. Nevermind the 02 sensors. You've proven that the IACV works. You'd have random engine shutoff problems with the EGR. The MAF is not usually the cause of these problems, so you are likely fine there too. Its always the TPS reset. I don't know where you live, but you'll have to find someone that can do that.

 

You can steal a TPS connector lock bar from the trunk dome lights

EV -1 type connector

Reminder to not have any anti-seize on the tip of the O2 sensor

There is a ground wire / shield ground to clean on the rear engine firewall that protects the O2 sensors return sensitive signal , a mini coax mire situation but the ground stud is a simple wire ( called a bottle rocket as I built a lot up in another field

I found that when I raised the exhaust down pipe up to the installed exhaust manifolds I damage the down pipe flange that the donuts seal to by the studs pointing down

Only option is to have manifold attached to head first , no swing out room

You ECT sensor values ( page 86 of the 801s doc ) are iffy on both old and new , the meter should be a 9 volt battery model that has the umf to power the sensor as it is a thermistor solid state device , the wiring diagram is also iffy on the exact wiring so try both polarities / lead swapping , gotta go to dialysis without computer

I think there was a demission on the butterfly to X for the idle stop screw , I see it now from Vee

There is a caution on the cheap IACs in reliability

There is a continuity test you can do but from the ECU for lack of room

Vee had a suggestion on giving the pins / blades on the MAF sensor a slight twist to make better contact and I don't see why that wouldn't apply to the IAC ( but the IAC are power wires )

So there are 4 square pin combinations minus the polarity duplicates

The ECU connector will be the Black one as these are power wires , the IAC should be warm during test and bang on IAC to see needle meter not wiggle ( digital will not pick this up )

 

This will break my heart a little bit if it is not DIY-able

So as you say with the butterfly valve and the set/grub screw, are you implying that the only way to get both values to work together — the .002 gap + the correct TPS voltage of .60 — is to have the gap set properly, then have a dealership reset the TPS?

I know this is slightly off topic and a defeatist sort of attitude, but has anyone had luck with aftermarket ECUs?

 

Just tossing something out there.....

Does the car have traction control?

I had a '95 as a daily driver for several years and had the usual high idle issues. A couple times I had problems keeping the two piece throttle cable in adjustment....and traction control actuator adjustment. Problems here will cause a high idle.

Cheers
DD

 

I might try and clean off the old IAC and throw it back in. Can’t hurt anything past what it is.

I have twisted the pins slightly a week or two before posting here after reading that thread with Vee too.

I will give the multimeter another try and do some proper research on how to use one - but yes! It is the 9volt model.

Off to work, gentlemen!

 

no traction control on the @WaldenFix car, but appreciate the suggestion!

 

It is not DIY-able. You need a device to reset it.

You will need the butterfly reset back to the 0.002" spec before you attempt to reset the TPS.

When the TPS is reset, it basically tells the ECU what the new voltage is at idle, so whatever voltage it's currently showing at idle, as long as it's within a certain range, will be absolutely fine. No need to restore it to 0.60 volts. The ECU has learned long ago that 0.60V isn't where idle is at anymore. That's where it was when it came off the assembly line, but no longer. I, personally, think it's a great idea to know what the voltage is just before performing a TPS reset. It could be useful in troubleshooting issues in the future.

Replacing the ECU will most likely still require a TPS reset, as the TPS is was matched too would very likely have a different idle voltage.

 

Error , Cafe Flyer covered he does not have TC

 

I would suggest to not jumper the ECT sensor as the resistance value goes lower as the coolant temp true temp gets higher in range of the closed loop 88 C

Better safe then sorry

Back from dialysis so let me decipher your table again

Are you reading the return wire voltage back to the ECU and check the ECU sockets that there are no missing 2nd pinching tabs that grips the ECU side blades

Red 14 to Red 31