Firstly sorry about another TB / high idle thread but I am starting to be at my wits end (and yes maybe that end is not that far) so bear with me

Car in question XJR 1995

problem description:

When ignition _on_ and engine not running tb potentiometer readings in OBD are :

• 11 to 94% when moving tb either by from the tb quadrant or by removing the cable from traction control end and pulling on the cable
• 11 to 89% when pressing accelerator pedal

When engine is running and pressing the accelerator pedal the car exhibits the classic high idle and corresponding the obd readings for are usually 12,5 to 14,5. Blipping the pedal usually helps

between 11-11,8 the car idles happily at about 680 to 690 at park so seems like the IACV at least mostly works

done things as of now:

• tb cleaning off car 2019, since driven ~1000km
• shortening the linear spring, might have helped a bit with conjuction of slackening cables until tb opening is only 64% max but car is sluggish
• heavier linear spring, helps but blocked the tb opening to 45% reading - car even more sluggish - changed back to original shortened one
• shortening of the top most / outter coil spring on tb (thanks Katar83 and Parker 2 for pointing this out), done just today
• cleaning and lubricating the linkage today
• disconnecting the cruise control linkage (both bellows and linear spring)

last three have been done today and the tb potentiometer readings are between 11-94% while ignition ON and engine OFF and to nobody´s surprise engine running the tb does not close properly and and the rpm´s hang


Now in my understanding IACV should not have any kind of direct influence on tb butterfly movement ? And the linear spring should be designed to ensure cruise control linkage return and as such not be mandatory for the tb to closing. Or in other words tb springs should be enough to close tb butterfly in normal conditions?

things to try:

• shorten the inner spring? (which as I am now thinking would make more sense than the outer spring)
• disconnect the IACV?
• buy a merc?

one thing that is worth mentioning that there was quite lot of axial play on the removed throttle body cap assembly as it is named in the TSB
fellow forum member nickdabs mentions that he also has similar issue here ( or at least that the coil spring shortening did not help ) and at the other jag forum someone else has the same symptoms. Unfortunately the person never revisited the issue.

Any ideas or pointers are more than welcomed at this stage. This has been most frustrating thing with the car yet and makes me not want to drive it.

 

I suspect you have one of two issues.

1. You need to reset your TPS. Typically done with a machine loaded with Jaguar software. The dealer usually can do it for an hour labor, even though it takes them only a few minutes. The butterfly should bottom out at 12.9% at idle. Perhaps the butterfly is going past where the throttle potentiometer likes to see it? You can adjust by hand, but it's a lot of work in the blind. Also, you're butterfly is supposed to be at .002 when closed. You can buy a feeler gauge to ensure that's where the butterfly gap is when at rest.

2. You may simply need a new throttle potentiometer sensor. Not cheap, but they are often the cause of weird problems like these.

 

The XJR is a bit different because the throttle shaft is horizontal, while the NA engines have it vertical. I had similar symptoms on an NA engine and it was the blowby from the crankcase creates sticky sludge and that gummed up the throttle shaft. I tried cleaning and it didn't work. However, that was spraying it with a cleaner and wiping it off, not really a deep clean. What I would try now is taking the throttle body and putting it in an ultrasonic cleaner and see if that makes a difference.

Ideally, the throttle shaft should be removed for cleaning, but there are no parts to put it back together, like old SU carbs can be rebuilt.

 

One of those TB boring services can probably make a custom butterfly, shaft etc.

 

On earlier threads, I described my painful journey through the high idle problem until I finally sorted it with a new throttle position potentiometer ( which didnt need calibrated, but worked straight out of the box).
After a prolonged period of non use, imagine my despair when I started using the car again to find that I was having 850 rpm tickover . Not as bad as the original problem when the idle could rise to an uncomfortable 1100 rpm, but definitely not right. Throw in occasional crank but no start from cold, and I was thinking of throwing in the towel.
When running a scan on my Autel scanner, I noticed that the ECT was reading 283 degrees before I had even started the engine from cold. Although it has been a warmer than average Summer here in Scotland, that seemed a little high!
Long story short, the coolant temperature sensor which I had replaced as a precaution when replacing the TPS had failed (note to self: aftermarket parts are so often junk). Putting the old one back in suggested a coolant temp of 57 degrees, and the car has started and idled perfectly at 650 rpm ever since.
On a related topic, when performing an OBDII scan for live data, my scanner shows "Module 51" and "Module 18" as options. On module 51, the throttle position shows 11% as the lowest reading, whereas on module 18, it shows a more intuitive 0.4% as the lowest reading. Does anyone know what the two modules relate to? Could one of the readings refer to the throttle position potentiometer on top of the throttle relaxer unit?

 

I've not seen the module description before, but an idle TPS reading of about 11-12% is typical. I don't think it is reporting the traction control uint position, but anything is possible.

 

Experiment by disconnecting the IACV and TC electrically (TC potentiometer and power supply off, IACV connector off); No effect on the TB sticking /hanging while engine running, engine off no issues. Idle logically was high all the time while engine running. Found one vacuum line from TB adapter to be free from other end and have no clue where it should be connected, maybe EVAP ? Plugged it with random hex bit, idle quality (not idle speed) got bit better so not totally futile poking around session after all.

Also experimented with random springs (and combinations) for the linear spring.; problem is to find suitable low spring co-efficient with high enough spring force at around closed position of tb butterfly to get it to close reliably but not to restrict tb to open completely. Too much spring tension seems to make TC to "open" and restricts TB opening at ~50-65%. And anyways this would be bandaid at the best, as the linear spring in my opinion is designed to return the cruise bellows and should not be needed for the TB butterfly closing in normal operation.

Most likely the bloody tb is just worn out (axle, bushing etc) and the air flow when engine is running some movement on the axle and thus causing the hanging.

Will (maybe) dismantle the TB next spring time, the car will go to winter storage soonish, and I really not feel like doing this now

Anyways car seems to be working pretty ok outside of the tb hanging, idle quality is nice and smooth, and in the spec.