About 1500 miles ago (which, a little sadly, represents two years) my '03 XJR developed a little idle surge. Not always, but frequently the idle will fluctuate from about 650rpm to about 550rpm. I don't think most people would notice it, but I did. Due to the slight use the car sees, I haven't really had the inclination to look into the problem. This morning on the way to work I decided I would make it a summer project this year.

I think the car heard me, because on the way home from work I pulled up to a stop sign, and just as I touched the gas pedal to turn onto a very busy street, I got red and amber warning lights, TCS and ASC not available messages, and FAILSAFE OPERATION warnings all at once. Oh, and the throttle pedal stopped doing ANYTHING. I literally had to idle across four lanes of traffic. Very exciting!

I turned off the car, waited a few seconds, then turned it back on. Same thing. I turned it off, removed the key, waited about 30 seconds, turned it back on, and it was like nothing happened. The rest of my drive consisted of two miles of neighborhood roads so I decided to chance it - no further issues.

At home, I plugged in an Autel MD802 and showed a P1229, throttle motor malfunction. I switched to live data, and most everything seemed more or less okay, at least expected. The only two oddities I noticed were that Throttle Track 1 and Throttle Track 2 disagreed by about half a volt - one was 2.8v, the other 3.2v and that calculated load was at 20% while *idling*.

I turned the car off to erase the code, just to see how quickly it would come back. When I restarted it, the idle went insane. It sat at a rock-solid 600rpm, then shot up to 1000rpm for a bit, then back down, rinse and repeat every 30-60 seconds.

I turned it off, popped the hood, removed the intake pipe, then turned it back on but didn't start it. With the hood up, I could hear the throttle body motor going berserk. Under hood, the throttle plate was vibrating at a somewhat slow pace, but vibrating and not just moving around. Turned it off, unplugged the connectors, and that's when I found it: The TPS sensor was only held in by one screw, and thus free to move around a fair amount. Adjusting the TPS causes the throttle plate to move around or stop, but under all circumstances the motor makes quite a buzzing. I am not sure if this is normal of symptomatic of a greater issue.

When I bought the car a few years ago, the previous owner told me the original throttle body had failed and he'd installed a remanufactured one. That was in '12 or so, about 6000 miles ago. That seems to be supported by this picture:



This all leaves me with some questions:

1. Does anyone recognize this reman sticker? I would like to yell at someone if at all possible,
2. Is there a procedure for resetting the throttle position sensor?
3. Does the throttle motor typically buzz all the time? Similar devices in other cars I have (Mercedes, Saab) are silent.
4. Does anybody happen to know if the throttle body is compatible between XJ8 and XJR? I have a spare from a running XJ8. The casting number on the venturi is similar, the part number on the TPS is identical. I don't see any other numbers to compare.

Any and all thoughts & suggestions are appreciated!

 

Swap the TPS from your spare and see if it fixes it.
The TPS should only move the throttle in a constant rev pattern. Not jump around unless the TPS is bad. Since you have the identical part, swap it and see what happens.

If not, there is a new Throttle body in your future.

The motor should not buzz.

 

What is the value of that versus just properly installing and adjusting the one that is currently there? I don't think the TPS is the issue so much as either its adjustment or the motor itself.

 

I've never known an XJ8 TB to work on an XJR engine and visa versa. I'd go for it though.

 

According to JEPC (Jaguar Electronic Parts Catalogue) although the TB's are different on NA and SC engines, the Throttle Potentiomer (P/N JLM 12074) is the same on 3.2, 4.0 and 4.0 SC models.

Graham

 

Couple of things to note on a TB swap - on the XJ8's there's a coolant plate at the rear for cold starting. On the SC version it's blanked - you'll need to swap the blanking plate.
The whine with ignition on is standard you can also hear the throttle plate click and whine on shutdown. There's 2 sensors on the AJ27 TB, throttle position and throttle pedal position, if either of these have worn tracks then an intermittent failsafe mode pops up randomly - I fought one a while ago and managed to make up a decent throttle from bits I had. There's also a long running thread/quest (I think in the XK forum) to locate new TPS sensors as they wear out and these codes come.

One thing I would be cautious about is realtime TPS setup with the calibration screw, tiny and I mean tiny imputs/alterations to the position wildly alters the rev's, so a steady hand is very important when the car is running and you're setting it up. Lock the screw, clear the codes plus a hard reset and see if it comes back.

Footnote - the way I found worn tracks was OBD throttle position, slowly pressing the throttle pedal and looking at the percentage rise and fall, loss of signal while doing this confirms the sensor is bad.

 

That is VERY interesting! I looked on ebay last night and their (unreliable) parts interchange did not distinguish between 8 and R so that gave me hope.


The two TBs I have - one on the car (but a reman, so not original) and the one I have on the shelf (which I pulled from an '03 XJ8) both look identical - at least from memory! Both have coolant passages. In the image below, are you saying the R version wouldn't (shouldn't!) have the two coolant ports?






I definitely have the clicks on start-up and shut-down, I've just never heard the loud buzzing before... I don't tend to sit around with the hood up and the key in the ignition... gave me some concern.



That makes sense - I *think* I had separate entries for pedal position and throttle position, and the "track" values in my OP were both from throttle position. All values were very linear so I think the sensors are good - not guaranteed though. *Definitely* the fact that the TPS was not locked down could be responsible for everything from the varying idle to the sudden failure... but honestly I suspect the idle issue is from perhaps the EGR valve or evap valve being a little off. It's never thrown a code for it, so I haven't messed with it *despite* how annoying it is.



Indeed! Is there a procedure for TPS setup somewhere? I would definitely prefer to try and fix the obvious issue before parts-swapping. Then at least I know what I'm working with.


Thanks!

 

I should have a picture of an XJR TB, I'll see if I can dig it out -
If you go for a swap do yourself a favour and buy a new throttle to elbow metal gasket, the amount of botch jobs and unmetered air leaks from these things is beyond a joke.

I'm not aware of a TSB on TPS setup, like I mentioned I do it on the fly. I take out any slack in the throttle cable and cable tie it, eyeball where the sensor originally sat and with finger tight bolts, start the car and see if the cold start drops to idle speed, if too high I feel pressure on my fingers on the sensor until the engine note changes either up or down as it turns (tiny, hare's breath) to get back to factory idle. Other forum members have done the same thing. You may get it spot on first time also.

Good practice is a hard reset following setup - the car loads the base throttle calibration file and it's good to go.

 

Man, eyeballing things like TPS sensors really makes me nervous. Every TPS I have ever dealt with has a base setting, either in ohms or volts and small variations can make major differences in drivability. I wonder if a similar Denso TPS is in use somewhere else, and I use that wiring diagram to get values from the good used TB I have.


Here is the TB on the car ('03 XJR):





And here is the one from my parts car ('03 XJ Sport)





Both are definitely wet, and in fact I clearly recall a coolant leak from the right-hand hose on my car when I got it... I bought the hose from Gaudin!


From the pictures it looks like the position of the left-hand hoses are in different locations - the XJR picture is from a while ago... I will double check that this evening.


I'd be very interested to see a different R TB... all the ones I've been able to find online are unclear as to their source, or they're from an angle where the ports are not visible.


Really appreciate your help on this....




Edit: Oh, yeah, you wouldn't happen to know the thread size for the TPS screws, would you? Looks like maybe M4 or M5? Seems I'm going to need to find one.

 

Some more information in case it helps anybody:

I found this:

http://www.bluejag.co.uk/Jaguar-XT-T...s/XT303-04.pdf

Which is about X-Types, but the TPS on the XJ is 198500-3250 and the TPS on the X-Type is 198500-3300 - I *suspect* they are pretty similar.

That PDF has a flow chart for testing the X-Type's TPS sensor, which includes:

I also found this thread:

https://www.jaguarforums.com/forum/x...acement-77560/

which has this anecdote:

I measured the terminals on my spare TPS and got:

1-2 closed 1.3k WOT 2.2k
1-3 closed .6k WOT 2.4k

Mine are spot-on within the X-Type's specs and Steve's from the above-mentioned thread are close. Obviously there is little to no science here, but I think these two sensors are bloody close, and the numbers in that X-Type TSB can be used to at least grossly set the TPS. So, I'm going to do that.

 

As you already mentioned, it is pretty common to measure the TPS voltage for the exact calibration, but I have never seen values for the x308.
However, your approach seem the right one, and you will easily find out once you see the resulting idle.

One word though, I always understood that the sensor tracks break down on intermediate positions.
If so, having the correct results at full open and full closed does not guarantee that the sensor is working for the whole range.

 

I'm ring rusty! it's not coolant lines as both SC and NA have them, it's the "Air Assist" line from the NA throttle body that needs the SC plate as seen in my pictures.

 

Because even the X-Type TSB shows a range of acceptable values, I suspect both of these cars have some sort of adaptation feature. That would be consistent with other cars I've worked on that use drive by wire systems. Sometimes it's automatic (on startup, for example) and sometimes it's commanded by the scan tool. I guess we don't know the answer to that one, although perhaps that's what happening when the TB clicks and whirrs on startup & shutdown? Kinda sucks that none of the Chinese clone scan tools directly support the x308. I've gotten a lot of use out of others.


Understoood and agreed! I'm just trying to find out how to put two screws back in my existing TPS so it's not flopping around anymore. After that, I will fully test its TPS. I *suspect* it's okay - I think the drivability issue/CEL is a result of the TPS physically moving around. *In theory* it's a 6,000 mile old reman... maybe someone forgot a split washer or threadlock. If that doesn't work out, I will swap my spare on - I just don't want to mess with it in exploratory surgery since I know it works, it came from a running car.

 

Okay, that's *real weird* - check out my pictures above... the NA version I have has *an* air assist port! It appears to be different from the one on the car, but it's not a big blank spot like yours. Maybe a running change? Very weird!

 

In the first picture lhs the SC body is stripped of both TPS/TPPS, but the main item is the part with the elastic band round it the air assist port blanking plate. The TB on the right is an NA unit with air assist plate and controller.
SC = no air assist
NA = air assist

 

Yep, understood - but look at the pictures:


Mine - pulled from an '03 XJ8 Sport:





Yours - from IDK.





My NA TB *has* that air assist port!

 

As it should, and you want to put it on the R? Just do the swap and cap off the assist valve.

 

Okay... so what do we think this thing that's on the car is for?

 

Have you had a look what that hose is attached to, removing the TB is 4 bolts and pulling the TB to see what you've got?

 

As you have a XJR perhaps you should look at this; https://www.carboncleaningusa.com/di...s/p1229-jaguar