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Discussion Starter · #1 ·
I have raised this before and didn't really resolve the issue. I have 3 Jags all with the 4.0L V8 engine. A 2002 XJ8 SE, A 2000 XJ8 Sovereign, and a 2000 XK8 Coupe. I drove the 2002 SE for about 6 years and it's now covered 250,000 miles - but it always was not very responsive to relatively small throttle pedal movements. I always took that as a function of the 'fly by wire' throttle control. If I booted it - it was off like a rocket! I decided that the SE wasn't going to last much longer so I bought a 2000 Sovereign. The difference in throttle response was immediately obvious - touch the pedal and it instantly speeds up. Not overly happy with the Sov, I bought a 2000 XK8 Coupe and guess what - it's got the same sluggish response as the 2002 SE!

I was advised to adjust out any slack in the throttle cable and I did that. It did have an effect in that the pedal requires less movement before the engine responds, but whilst moving along, pushing the pedal down has little effect until quite a lot of travel has been taken up. It's almost as if the throttle cable is made of elastic! The year 2000 car though feels like pedal is directly connected to the engine.

I've done a bit of reading up and it turns out that (except in Limp Mode) the actual throttle position is totally controlled by the PPS (pedal position sensor) feeding values to the ECM, and the ECM feeding a voltage to the throttle servo motor in the throttle body which moves the throttle disc, the position of which is then monitored by the TPS - which feeds data back to the ECM. (Well that's my interpretation of the system). The shaft passes right through the throttle body, but the throttle cable end does not open the throttle disc - the servo does that. In other words the shaft is separated at the cable end - although it looks like a solid rod. The only time the cable directly controls the throttle disc is in Limp Mode - and that is restricted to 30% opening.

So the sluggish response could be caused by a sluggish PPS, a sluggish servo drive system to the disc, or the ECM.

Now to further clarify the problem - my 2002 SE had a Limp Mode event a few years back. I ended up buying a replacement throttle body (2nd hand) and put that on. Fortunately it worked and solved the Limp Mode problem - but the car was just as sluggish as before (or to be more correct I didn't notice any better throttle response). This might rule out the possibility that it's the throttle body. So is it the ECM?

Both the year 2000 cars were registered in March 2000, and are therefore probably Nikasil Linered. (The year 2002 car is of course steel linered). Jaguar started steel linered engines in August 2001. However, I don't know if either were replaced but, seeing as British petrol was low Sulphur from 01/01/2000, then neither should have a problem. For now I will assume both engines are fine mechanically.

I'm wondering if I can take the ECM out of the Sovereign and put in the XK8. Alternatively, most car ECM's have a removable EPROM with the engine mapping on. Does anyone supply these?

By the way - the word 'sluggish' is relative. The XK8 and 2002 SE only show up as 'sluggish' when you've driven the Sovereign.
 

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Is the XK8 equally "sluggish" in sport mode? Of more relevance, is the Sovereign standard or has it had any tweaks done to it by previous owners?
 

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Discussion Starter · #3 ·
The XK is permanently in Sport Mode - and why would anyone 'tweak' a Sovereign? ........... It is a thought though ........... I wonder if the Sov has a different chip in it? I'm going out later so I'll lift the bonnet (of both) and see if the ECM's are accessible (I usually avoid anything with wires sticking out!) No doubt there won't be big label with ECM written on them - are they easy to identify?
 

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Discussion Starter · #4 ·
I don't want to dilute the thread but I have just noticed in the XK8 'Instructions' that it says -

"" The throttle body also has an accelerator pedal position (APP) sensor consisting of twin potentiometers which
provide separate analogue input signals, proportional to driver demand to the ECM. As a further safety feature the
two potentiometers have different input/output characteristics for unique signal identifications and any corrupt
signal between the expected outputs will cause the ECM to switch the throttle to limp home mode. ""

(I called the APP a PPS in my thread).

Whenever we talk about Limp Mode events we always home in on the Denso rheostat on the servo end of the throttle body. This suggests that it's not the culprit - it's the APP at the cable end (on the cable drum). Has anyone ever replaced just the Denso jobbie and cured the Limp problem?
 

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Jim, I have noticed the same thing on my XK8. Don't get me wrong, it is fast and very quick from a standing start but once up to a cruise, say driving along at 50-60 mph in 5th gear, if I want to overtake, I need to really boot the throttle to the floor to get the 'kickdown' to work, then the car takes off like s*** of a stick. If I only press the throttle pedal a little way, there is no kick down and the car picks up speed very gradually. On previous Jags I have owned I seem to remember the kickdown coming in more readily than it seems to on the XK. However, the handbook does say:

Kickdown is operated when the accelerator pedal is pressed fully down, beyond the normal operating spring pressure, to provide maximum engine performance. Kickdown is used in circumstances where rapid acceleration is required, such as when overtaking. Kickdown causes the transmission to change down to the lowest gear possible to achieve maximum acceleration. The gear engaged depends on the road speed at the time of kickdown. As well as shifting down, the gearshift points are extended to give greater performance. This mode is in effect for as long as the pedal is pressed fully down.

So this would seem to be normal operation.

You may also have read on another thread that I too am having issues with the throttle position sensor and limp mode. I am intermittently getting limp mode with fault codes P1121 and P1122 which both indicate throttle position sensor faults. I have tried cleaning the electrical contacts and also cleaning the throttle body itself (which was surprisingly dirty by the way) but I am still getting the problem. I have been quoted almost £1,600 for new throttle body so I am about to get it changed for a second hand one by a fellow forum member Trevor at Jagspares in Penge (Catkiller) and hopefully this will solve the problem. Trevor was saying that he has changed 2 of these in the last 3 weeks and it seems to be a very common problem. The TPS fails and the solution is to replace the TB. The job will be done next week hopefully and I will report back with an update on how it affects the way the car drives.
 

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Discussion Starter · #7 ·
Rick - I am tracking Ebay for a suitable 'spare' TB because, having experience what happens, I don't want to get caught out again. It's worth £100 just to have one 'on the shelf'. If needbe I'll buy a whole MOT failed car and take the TB off it.

As for the performance - if you can drive your XK get up to (say) 40mph and them 'waggle' the accelerator about 1/2" up and downwards. On my 2002 XJ and the XK there is no response - but with the 2000 XJ it accelerates. Somewhere there is a lag in the system.
 

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Jim, it might be worth you speaking to Trevor at Jagspares. He laughed when I asked him if he had one in stock - he has 100's of TB's in stock - for £125 each.
 

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Discussion Starter · #9 ·
Now that is worth knowing rick. I might just call him.

P700 - I'm surprised at that. Mind you, with 4.0L perhaps it shouldn't need it. What is so galling is that my 'drives like a mattress' Sovereign goes better than my 'super sporty' XK8! It's really bugging me.
 

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Discussion Starter · #10 ·
I've done a search in here (with no success) for a pic of the inside of the V8 ECU (or ECM). Has anyone ever seen a pic of a unit with the lid off? I'm asking because I'm wondering if the main chip is a removable EPROM - so much easier to unplug that than move/change the whole unit.
 

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It probably uses FLASH memory and although I have not seen in side one, I would be willing to bet it is not unplugable.

I have bought lots of stuff from Trevor, he is a great resource for spares.

Sluggish, yes Kim's is too when just cruising along, a blip of the throttle does disappointingly little, I think it is because the rpm is so low when just cruising along the bhp is just not there without dropping a couple of gears.

Maybe a sprint booster would help, it changes the transfer function of the throttle position sensor, translating a light blip into a stamp on the throttle. Don't know if they have one for the XK8 though.
 

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Same here, sluggish to pick up when cruising. Almost as if the engine goes into "sulk mode" unless given a change down to "lift her skirts"
Makes a diesel feel perkier with their torque peaking at those sorts of speeds!
It is almost as if the throttle body motor is closing the throttle in response to it being opened.
Would be interesting to interrogate the real throttle opening, timing and fuel trims.
Anyone have the torque and BHP curves available for the 4.0?
 

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Discussion Starter · #13 ·
Sulk Mode - I like that !! One thought I have had - 2002 SE and the XK8 are both 'sports' jobbies, whereas the Sovereign is a 'standard' car. I wonder if the gearbox is different (or the engine mapping) different because of the ratio differences.
 

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I would never describe the XK8 as 'super sporty', more like a big engine in a pretty dress.

Now an XKR is a sporty car in a pretty frock for sure!
 

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Jag ECU may be like many others, where they are coded to the SCU, therefore just swapping over an engine ECU will cause the immobiliser to stay "on" Would need to be confirmed.
 
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