OK, here's an interesting one.... get into the car (1997 XJR) this cold morning, starts fine, idles fine, press brake which allows me to shift into D, car pulls ahead on driveway under idle, then when I press the accelerator to pull out of the driveway into the street... nothin', nada. Good thing no traffic was coming!

Fortunately for me even though I rolled out into the street at idle (while repeatedly pressing the accelerator with what was I'm sure a very confused expression) the car had enough torque even at idle to get back into my driveway So hey, that's an endorsement to the torque of the 4.0 liter 6!

Since I had to make an early meeting, and it was 30F outside, I parked the car and jumped in the Honda to get to work. So I didn't have a chance to do any troubleshooting yet.

Here's what I do know though:

1. The car has never exhibited this behavior before
2. The car has been sitting for a couple weeks due to the brake light switch failing
3. I replaced the brake light switch this prior Sunday
4. This week was our first really cold days of the winter, hitting the 20s (F) for the first time last night

Symptoms again:

1. Starts fine
2. Idles fine (not high, not low)
3. Brake lights work and allows shift from park to drive (any gear)
4. Pulls away in drive or reverse under idle fine
5. Accelerator can be depressed with no undue resistance, feels normal
6. Accelerator has no effect on RPM
7. I have no CEL or other warning lights lit, but haven't put a code reader on it yet

I did some quick forum searches and didn't find this exact scenario.

This looked interesting and possibly relevant, but no replies were made:

https://www.jaguarforums.com/forum/x...roblem-138513/


Of course the first thing I'll check is to see if I've somehow disconnected the accelerator cable from the pedal arm under the dash while I was under there replacing the brake light switch

I'll also of course try to verify which cables are moving or not from pedal to traction module and from traction module to throttle body.


However, if that isn't it.... thoughts on what to check in addition?

I thought that I too had read that the X300 was essentially "drive by wire" in that the traction control actuator actually moved the butterfly in the throttle body based on the cable and position sensor from the accelerator cable? Can someone refresh my memory how the traction control actuator actually works in concert with depressing the accelerator pedal and ultimately moving the butterfly in the throttle body?


If that is true, could there be an issue with the traction control actuator:

Brake Actuator Cable Assembly-Traction Control - Parts For XJ Series from (V)720125 to (V)812255 (X300) | Jaguar Heritage Parts UK


And if so, is this the unit that occasionally has the wiring harness issue that requires some reinsulating/soldering, or is that the ABS module?


Or could it be something like a TPS issue on the traction control actuator, throttle body or another sensor where I am indeed moving the TB butterfly, but the ECU isn't actually allowing the injectors to add more fuel, etc?


As you can see, grasping at some straws here until I get a little more info. I'll update once I know more.


Any other thoughts/tips are appreciated to help with the troubleshooting.


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The PDF below answers my questions about how the traction control module and throttle cables work. I knew I saw it somewhere.

Didn't get home until late, so no additional info to add yet to assist with diagnosis.

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Do you have an OBDII reader? If you have one of the better ones it will show throttle position, which is read off the TPS. Hook up the reader and put your foot down (engine off, key on) and see if the readout changes.

 

Back in the day we all fought the throttles day-in-and-day-out.

TSBs for the throttles/

bob gauff

 

I would start by getting someone to press the accelerator while I looked at the throttle to see if it is actually operating. If not, with engine running, I would operate the throttle manually from inside the engine compartment and see if revs increase. As you say, the cable may have come undone somewhere, but aside from that, I would suspect the cruise control.


On my donor car, when the engine was running and cruise was engaged, the revs would climb rapidly and very significantly. They are definitely connected, but my hunch would be that your problem is a simpler "mechanical" issue.

 

Thanks for the tips. Once I get some sunlight and time I'll definitely check the simple mechanical options first.

Good tip on the OBD tool as well.


I'll let you know what I find once I get a chance to take a look.

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Easy on a normially aspirated car, but Al has an XJR and the throttle is buried under the supercharger and impossible to see. It's actually easier to get at the throttle from under the car when it's on a hoist. I think the entire engine was built around the throttle body!

 

My bad, didn't realise that. I've had cars like that in the past!!

 

You can plug an OBDII scanner into the diagnostic port and actually WATCH the TPS on most units with a DATALOGGER function.

It should read in % of throttle opening.

bob gauff

 

Having this exact problem.

Or rather, had it. It's been getting down to the low 20s the last few nights here, and found the car fired no problem, but accelerator had no impact on RPM.

After idling for around 20 minutes, I was able to get some change in RPM, but it is slow as molassas. In neutral, drive and reverse, flooring it will result in a very slow (i.e., 5 - 10 seconds) rise in RPM from about 1k to 2,500, at which point it will rapidly rise towards redline.

I believe the throttle goes into the cruise control module, which then actuates the throttle. It's almost as if the cruise control module is frozen and/or impaired with some half-frozen liquid that's impeding the throttle from smoothly opening and closing.

Any ideas here to get the thing unstuck without towing into the shop? N.B.: the car had been sitting for about a week through the cold temps before I tried to start it two days ago.

ETA: this seems to be the long-term fix, re-posted from Bob Gauff above: https://www.jaguarforums.com/forum/a...old-engine.pdf

 

Yeah, I'm expecting to be using my mechanic's mirror

Apologies that I've had no update yet, but been working late and the weather is less than ideal.... so I haven't gotten under/into the car yet to test.

Will update once I know more, which will probably be by Saturday afternoon.

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Well that DOES sound very interesting, exact same symptoms and environment (sat for a while, COLD)..hrmm.

One of the tests I plan to do is warm up the "Traction Control Actuator" with a hair dryer if nothing else helps to see if the problem is temperature related.

With that in mind, does anyone off-hand know which fuse locations may impact the "Traction Control Actuator"?


On that note, although I read the document I posted above that describes the "Traction Control Actuator", I too wonder if the two cable "cams" in the "Traction Control Actuator" are directly connected(or the primary cam pushes the secondary with a "tab"), or if they are completely independent and if the actuator actually moves the second cable actively and independently (via the electric motor on the actuator) based on the position sensed of the accelerator cable potentiometer. Anyone know for sure?

If the actuator actually moves the second cable to the throttle, then any malfunction in the actuator could cause this problem, much like a fully "drive by wire" system.

But the document seems to suggest that the "Traction Control Actuator" only backs off throttle under traction control events, so that would seem to suggest that the "Traction Control Actuator" has no active role in opening the throttle.

Without disassembling a "Traction Control Actuator" to see, I really can't say of course.



BTW, I don't think the "Traction Control Actuator" pulls the throttle for cruise control. There is a vacuum solenoid on the throttle body for that as I recall??


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Correct.

 

Well, my son and I go out this morning to troubleshoot the problem.... and upon starting the car the throttle was still initially unresponsive for about 5 pumps..... then all of the sudden starts working, with no change from us. It seemed a bit more sluggish than normal but then seemed to improve a bit.

So I don't know what was going on. But it's better for now. I'm not sure how happy I am about that scenario because now I won't feel confident about it happening again, perhaps while driving as I don't know what the cause was.


I still feel like the throttle is sluggish on this car compared to all of my others. Not having ever driven another X300 I don't really know how "snappy" the throttle is supposed to be, but there does seem to be a definite slight 1/2 sec lag between accelerator being pressed and the throttle/RPM responding.

For example when I even just give partial throttle to my Buick (3.8L, supercharged), the RPM snap right up and it will spin the tires even if already rolling, the XJR seems to take a split second to think about spinning up and I've almost never been able to spin the tires unless on wet pavement with trac control off.

I'm not sure if my throttle response is normal or not.... although the old-time tech at the Jag says it's normal & OK so *shrug*

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Clean your throttle plate. I know it's hard to get too. The same thing happened on my 97 xjr. The oil from the supercharger gets into the intake a little at a time and gums up the inside of the throttle body. Hope this helps

 

Thanks, when you say the same thing happened, do you mean the no response to the accelerator?

But yep, cleaning the entire intake is on my list this Spring.

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Yes absolutely no throttle response. Car would start go into gear and move just like it was idling. A good way to tell, if it happens again is shut the ignition off press and hold your accelerator to the floor about 4 or 5 times. You may hear a click. Then start car and see what happens. Usually starts showing up in cold weather

 

Thanks, really glad to hear I am not the only one that has experienced this.

While mine is working again now, the accelerator does show a little initial resistance now then "let's go" and presses smoothly to the floor, but only when running. When not running the accelerator does not have that initial resistance.

I'll just have to put cleaning out the intake tract on my Spring cleaning list, which also includes having my injectors rebuilt and installing a new fuel pressure regulator to refresh the whole air/fuel system.

 

I did a little research for my friend who's having the same issue. You know anything about the sensors(supposedly there are sometimes 2) near the accelerator

 

Melissa ,

Inline 6 or V8 ?

The later V8 ( X308 ) same chassis from my understanding has sensors at the gas pedal

Inline 6 does not so the TPS is on the butterfly

Do not remove or adjust the TPS just yet