Folks,

As part of Rose's recovery, some time down the road I have to tackle her high idling issue which I understand is not uncommon to X300s. I have learned from previous discussions here in this Forum that it is imperative that TPS Adaptation must be conducted, as I have replaced the ECU during the auto-to-manual modification, and this may have disturbed the original match between the ECU and the Throttle assembly. However, all adaptation procedures (I also need to do O2 adaptation as I have repaled the two O2 sensors) have to deferred till I have access to dealer level diagnostic tools with IDS and SDD capabilities. Meanwhile, I understand from past research that high idle can be cured, often by replacing the idle speed control valve (ISCV). Here is a detailed write-up by an earlier X300 DIYer: Faulty ISCV can cause high idle.

I tried to identify the part and get one new, but, in the process, have run into many different names and parts number. I found Idle Air Control Valve (IACV) in Jaguar Classic Parts shown under sub-category "Idle Speed Components", where IACV carries Parts No. LNA1500AB, found in both the right AND left Rail, implying that it is only available on V12 model? See picture and diagram (Item 12) below.







I saw no mention of IACV in non-V12 X300s. Instead, I came across mention of a "Stepper Motor" (JLM12185). I attach a picture of the stepper motor and the location diagram (Item 2).






My guess is the "Stepper Motor" is in fact the Idle Speed Control Valve for non-V12 X300s. Just want to validate my guess before I decide to acquire a new spare towards addressing the high idle issue.

 

I can confirm that the JLM12185 is the correct part for the AJ16 engine.

Prepare to be disappointed if you think this will fix your idle issue. It might. What I have found is that replacing the IACV screws up the idle worse than any TPS issues I've had in the past.

There is a way to manually adjust the TPS. It's supposed to read 0.6 volts when closed. You'd have to remove the throttle body (give it a thorough cleaning, which it'll need) and then if you open up the mounting holes a little, it should give you enough room to adjust the position of the TPS, resulting in the proper voltage.

The key here is not tightening down the TPS screws all the way, just enough to allow you to move the TPS under a lot of force. Then when you reinstall the throttle body, you can slide the TPS until you get the proper 0.6v reading, then tighten the screws, in the blind, upside down, the rest of the way down so it not longer moves.

I'd go that route before the IACV route.

 

The first one is for a late V12. The second one is for an XJ40, although the stepper motor is the same between XJ40 and X300. (I think - it looks the same. With my patent writers hat on, I'd call it "substantially similar").

In your XJR, it's bolted onto the throttle body. Those small bolts have loctitie on them, so unless you heat the bolts first to break the bond, you'll shear off the bolts if you try and undo them (ask me how I know...)

 

Here are some photos of the XJR throttle body, you can clearly see the idle air controller.

The XJR throttle body is different than the normally aspirated one, but the idle controller should be the same.

 

Be aware that the IACV is retained by 2 small long screws that have THREADLOCKER to secure the screws. You might snap the screws off if you try to loosen without heating the area where the screws are threaded into the housing.

I use a SMALL butane torch with a pinpoint flame to melt the threadlocking compound before I attempt to loosen the screws.

I collected boxes full of old throttles from the other mechanics at the dealer because they had broken screws still in the throttle body. A new one from the parts dept for $800.00 and car was back on the road.

I carefully removed the broken bits and cleaned the throttles up.
I put a dozen or so up for sale on eBay about 20 years ago and sold them all.

BE CAREFUL with the small screws.

bob

 

Many thanks for the heads up. I have to pass them to my mechanic so he would know what challenges he will face. He plans to remove and tackle the supercharger and air ducts some day to service and fix any potential problems there (irregular noise coming under is a symptom of issues), and to access, clean and manually adjust the throttle body which now has periodic hot idle ranging from 1200 to 1500 rpm (c.f. 800 rpm when idling cold), which is possibly the cause for high CO and high fuel consumption. I know that high idle can be caused by many factors, but the underside of the supercharge and the supercharger itself have yet to be opened up for a deep clean and fix. I have purchased a supercharger servicing kit, temperature sensors, various rubber hoses along the air duct pathways, and a new "stepper motor" to do it all in one go. If these wouldn't fix the high idle, then hopefully the O2 and Throttle Body adaptation would.