High Idle XJR
 

My beloved XJR has always been blessed with a smooth idle, at precisely the textbook speed. Recently a pronounced intermittent misfire developed, culminating in a couple of refusals to start.
I read the codes, which produced something of a list, including Crankshaft position sensor, misfire, and coil related codes.
Having not hitherto replaced the Crankshaft position sensor, I cleared all the codes and fitted a new Crankshaft position sensor. Bingo!, all misfires gone, the car pulls like a train, perfectly smooth idle, and no codes.
The problem? It now idles at around 950 rpm in gear, 1350/1400 in neutral. I did not touch anything other than the Crankshaft position sensor, and was working from the side of the engine bay away from the throttle body, MAF, supercharger etc
Any ideas guys? It idled higher from the first turn of the key after fitting the sensor.

Check the fuel trims - hot engine, parked. Idle & about 2500rpm.

Most likely needs the TPS reset. It can be done manually, by loosening the screws to the TPS and rotating it slightly.

A one-hour labor bill by a Jag dealer will likely be your best solution.

I'm not sure why a new crankshaft sensor should affect how the TPS is read, bit I have had a similar issue when replacing the Idle Air Control Valve, and it always seems to require a TPS reset to get the idle back to normal.

Doesn't appear to make sense but may be worth trying - but do the trims first as it's almost free & DIY.

Could be you have knocked a vac pipe off somewhere causing an air leak - as per @JagV8 checking the STFT may show this.

Also, check that the throttle is actually fully closing at idle.

Also, as it's straight forward try putting the old CKPS back on and see if the idle returns to normal. I can't see any reason it would (but you never know!).

Thanks guys. I will pursue these and report back.

Other then the common mechanical interference not allowing the butterfly from retarding to the idle stop

If you put your hand on the butterfly shaft tower and rotate it to the mechanical stop the TPS should read 0.60 + or - 0.02 volts DC on the Green / Yellow wire with the key to the on position but engine not running . This Green / Yellow wire is the middle wire on the TPS connector but is underneath the TB but can be tapped into as it comes over the fuel rail in the wire bundle . A scallop cut in the insulation will work

Another way to read the TPS is through the ECU connector at socket Red 12 and Red 7 and you'l l be looking at a resistance value

As you rotate the butterfly slowly through it's full range you should not see a bip on your meter indicating a open spot on the wiper arm , This bip would going to zero voltage or high resistance depending on how you are reading it

Once the ECU sees a bad spot on the TPS it rejects the signal input and defaults to a less then optimal set of ECU maps to regulate the engine

You had a TPS CEL fault to indicate this

See page X

http://www.jagrepair.com/images/Trai...20-%202000.pdf :icon_dance-disco:

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Editing

UPDATE
Having been distracted from the XJR for some time, I started her up this morning for the first time in about a month. Started first turn, and ran sweetly, but still too high a tickover.
I attached my code reader to look at the live data. Car quickly switched to closed loop on both vanks, and showed a STFT of 24% on both banks, with LTFT at -1.6% for both banks.
What caught my eye was the throttle position which showed 10.8% with my foot off the pedal. Pressing the pedal produced a corresponding movement on the graph, but I am now wondering what the throttle position should read at Idle. 0%? 5%? 10.8% seems high and might point to the need for a TPS reset?
I have checked under the bonnet, and cable is not snagged or too tight, and as I mentioned before, all I did was change the CKPS.

Agree with @Vee that Throttle Position looks OK.

I'm still thinking Air Leak. STFT at 24% (that's the max it can go to) on both banks shouts air leak to me.

From what I have observed from others the TPS is close in range for beng on the idle stop at the 13 % reading , never seen 0 %

Vacuum leak checks :

Cap the line on the top of the intake manifold # 3 cylinder going to the brake booster , this will limit your brakes in test drive . I think it's referred to as a banjo vitting with a squirrelky seal . Maybe a ball of interference tape

Cap the line on the bottom of the intake # 3 cylinder going to the EVAP system below the air filter , I found one of the EVAP hoses missing on mine

Cap vacuum line behind fuel pressure regulator

Editing

The EGR valve has a error followup position sensor if it is stuck open in a uncommanded position , It is in your ELM - 327 device as a PID

https://cimg7.ibsrv.net/gimg/www.jag...fdb0e81d5d.png

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The Idle air control valve does not have a error followup position sensor so you would have to read with a meter to see if you have a open pole winding on the 4 wire stepper motor , To remove it on the supercharged model and clean the poppet valve is a chore from my understanding

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Further Update
I took the car for a longer drive, featuring a bit of an "Italian tune up", good fun, but no difference to idle speed. I noticed the throttle return spring was rather flaccid, and renewed it. No difference, but while working on changing the spring, noticed that the throttle shuts with a metallic click sound, which sounds quite crisp and not gunged up.
With the engine running, I set about spraying easy start on all of the joints around components on the inlet side, in the hope that revs would rise when a leak allowed easy start into the inlet tract. Nothing.
Ran a diagnostic scan, during which the idle returned to normal, both in and out of drive. Tried a few blips of the throttle, and all was well. Switched off, and started to pack up my tools, happy, but not convinced. Once packed up, i decided to start the engine again, just to check that everything was still OK, and you can guess the rest!
In the ensuing bout of depression, I started the car several times and each time it did the same thing, although I did notice that as the engine caught, it initially idled at about 950rpm, and gradually just edged up to about 1100 in a series of small uniform progressions over the course of about 10 seconds. It seems clear that the elevated level of idle is being commanded, rather than caused by a sticking throttle or an air leak. In support of this, the high idle is stable and always at the same RPM.
What would cause a higher idle to be commanded?
My throttle position is now showing as 13.3% with my foot off of the pedal, which I understand is correct. The scanner shows the car to be in closed loop, with short term fuel trims identical on each bank (+24,2%) and the long term fuel trims also identical (-1.6%). I am suspicious of these fuel trim readings as they have not changed, and I am wondering whether my scanner isnt able to read the trims, although it has in the past. That said, when I had an issue with high STFT in the past, due to an air leak into the exhaust ahead of the O2 sensors, the idle was not affected.
Running out of ideas!

An idea.....

Just for the heck of it....

Before getting any deeper in technical stuff......

Slacken the throttle cable between the TB and the Trac Control unit. Does the idle drop?

I had (various) high idle problems with my XJR ("The Tormentor") over the years. In one instance (long story short), slackening the cable solved the problem. It may have been a cable problem....but I think it may have been a TC problem where the TC unit was interfering with the action of the throttle cable.

More recently, on my V12 car, replacing the throttle cable solved a persistent high idle problem...after investigating lots of other more interesting possibilities

Cheers
DD

Eureka!!
i think I may have resolved the idle issue on my beloved XJR.
Having convincesd myself that I would have to strip down the induction side to clean the throttle body and check out the throttle position sensor, I have been putting the job off for ages. The bullet was finally bitten this morning, and things started well, forgive the pun, as the car burst into life on the first turn of the key, despite not having been touched for about 6 weeks.
having momentarily run at 1000 rpm, it picked itself up to 1500 rpm and settled, so the problem was alive and well.
I had made a mental note to try a couple of the helpful suggestions which had been made on the forum first. Firstly, I disconnected the battery, touched the terminals together with the ignition switch in the run position. No difference.
Next, I refitted the original crank position sensor, which I had cleaned up.
Problem solved!
i washed and cleaned the interior of the car to celebrate its return to frontline duty, but have been reflecting on the experience.
I didn’t buy the new sensor from Jaguar classic, but from a reputable supplier who often features on this forum, and it wasn’t cheap.
I am no electronics expert, but know how a Hall effect sensor works, and have concluded that the signal from the new sensor could not have been “strong” enough for the ecu. Seeking a set value from the sensor, it would have commanded more fuel until the signal from the sensor conformed with the value the ecu believed corresponded with normal idle speed.
Following on from this, I am wondering whether it would be possible to wire in some form of signal amplifier between the sensor and the ecu, which would enable idle speed to be user controlled by duping the ecu.
I am also thinking about the impact the wrong signal would have had on fuelling throughout the Rev range, but that remains in the too hard basket for the moment.
Again, thanks for the suggestions guys, most particularly the most logical and obvious one about refitting the old sensor!