Hello all. I have a 2006 Jaguar XJ8 and while the car drives beautifully, the gas pedal seems a bit touchy when starting from a stop. I barely touch the pedal and the car wants to lurch forward as if it were in a race. Is there an adjustment somewhere to tone this down a bit or something I should check?

Thanks.

 

Are you in Sport Mode. That adds to the "touchiness."

 

Nope. Not in Sport Mode. It's always been a bit edgy (i.e. starts a bit fast from a stop), but I've just now decided I'd like to see what I can do about it. The car provides such a smooth ride that I'd hate to jerk around my passengers (and myself) from a stop sign or light!

 

There is a potentiometer arrangement which is integral with the 'gas ' pedal and

is similar to the TB position sensor.

Both of these involve a track and electrical contacts.

Possible fault developing with one of these components?

Maybe try used 'gas' pedal?

It may be possible to reset TB sensor- "Brutal" will know how to do that.

Just thought also clean TB in case butterfly is sticking.

 

Disregard

 

Mine jumps more than I was used to with my previous car, I've learned to adjust my foot for it. I never thought of it as a problem, just an adjustment for me to the car's nature.

 

In my experience, the normally-aspirated cars are touchier from a stop and feel quicker off the line than the supercharged cars. I haven't found anything in the Jaguar documentation to explain it. The accelerator pedal position sensors are the same for both cars, but the throttle bodies & throttle position sensors are different. I think I recall that the gearbox programming is also different. It may be that the N/A cars are programmed to start in 1st gear while the S/C cars are programmed to start in 2nd gear.

The throttle position is supposed to be calibrated by the ECM at each ignition cycle. When the ignition is switched on, the ECM commands the butterfly valve fully open and fully closed, relearning its full-stop positions.

The APP sensor contains two elements that are monitored by the ECM for range and plausibility. There are limp-home modes should one or both elements fail, but as far as I know there is no calibration procedure given in the documentation. Maybe SDD can do it. If someone knows we would be happy to learn about it.

Brutal once posted this procedure for calibrating the TPS:

"You dont need IDS or to go to the dealer. You pin the signal wires with a dvom and turn it(the TPS) to get a .6volt reading at idle. And yes you dont need the engine running or the throttle body on the car. Just the sensor hooked up and the key on, and throttle at the idle position. You can turn the TB over to access it easier."

I like meirion1's idea about cleaning the throttle body. All the ones I've inspected were moderately to very dirty. You can do a reasonable job of cleaning it without removing it, by using throttle body cleaner spray, an old toothbrush and lint-free paper towels. You have to hold the the plate open with one hand and clean with the other. But to do a proper job, it's better to remove the TB, which is not too difficult on the N/A cars. Much more of a job on the S/C cars.

Note that Jaguar advises against cleaning the TB. (!) I am attaching the TSB for your reference (courtesy of motorcarman). I just try to take care not to spray too much cleaner on the ends of the shafts which could wash away the grease on the bearings.

Cheers,

Don

 

My 2006 XJ8L is the same. Very hard to avoid "jack rabbit starts." Wants to accelerate with a lurch, no matter how carefully I push the accelerator. Just barely tolerable. I have wondered whether it was a "bug" or a "feature." I have wondered whether the TPS or some other sensor could be faulty.

 

My previous XJ8, a 2001, was not nearly so touchy.

 

Guys,

When I had my04 XJ8, I would notice a good difference in throttle response in the cooler months. The car felt more aggressive and powerful. And that was here in FL where it'l drop down to 50s. I can imagine in northern areas where its cooler you would feel it even more. The fact is, our cars adjust timing depending on IATs. So maybe you are feeling some of the power that comes with the cooler temps.

+1 on adjusting to it. When I drive my wife's BMW for a while then go back to my Jag, it'll take me a bit to remember I have to baby it. In order to avoid the jump when leaving off the lights, I let my foot off the brake pedal and let it start moving on its own a bit before I ease into the gas.

 

No answers just a thought. If TPS1 is set too low (ie below 0.6v) will the throttle quickly try to get to that as the peddle is pushed? This would cause the rapid initial acceleration. If the ECU does check for limits each time it may assume bottom end is not below 0.6v. Drastically out of range should trigger a code however.

 

What I think is going on is this: Somewhere in the software that drives these cars, there must be a table that relates throttle position sensor reading to throttle body actuation. In stock format, in this table small values of throttle position sensor evidently translate to relatively large values of throttle body actuation. "All" that needs to be done, is to identify where that table is, and alter its contents so that small values of throttle position sensor translate to more modest values of throttle body actuation. Is anyone knowledgeable enough about the Jaguar software to know whether this idea has merit, and whether such a data table could be edited?

 

If there was a problem with the TB signals and voltage you would definitely get codes and even "restricted Performance" ... trust me I went through all that when I upgraded my TB and had to calibrate the idle position and tps angle.

There are very few in the world (probably less than 5 total) that have cracked the codes on our ECU's in order to "tune" or increase performance on some of the R cars for these model years. Slim chance to none that someone will go through all of that work of searching and re-writing the code to do something that's never been done before just to de-tune just one clients car. I guess anything is possible for the right amount of $$$$ though right?

If its really a big issue, take it to a shop to inspect as maybe there is something wrong with the TB or connections. I doubt it though. Otherwise just take it easy on the gas pedal. That's free.

 

To add another variable - a slow acting MAF sensor would cause the same issue. The throttle would react to get the airflow it is expecting and give the rapid start. I would think it would catch itself and then one would feel a drop in acceleration as the MAF provides a proper value.

 

"There are very few in the world (probably less than 5 total) that have cracked the codes on our ECU's in order to "tune" or increase performance on some of the R cars for these model years. Slim chance to none that someone will go through all of that work of searching and re-writing the code to do something that's never been done before just to de-tune just one clients car."

I don't suppose any of this handful of experts looks at this forum !!! Besides,if this code is cracked, why, there might be a dozen or so owners who would take notice and benefit .....

 

I can adjust the throttle responce for you in ECU parameters. There might however be an underlying cause to the issue you are having, We can do some logging of parameters to see if can pinpoint, otherwise adjusting sensitivity should not be a problem.
On my own car during tune development and other modifications, there were several occasions where I had to turn down the sensitivity as it was very touchy and I was afraid if I would let my wife drive it she could be in trouble ☺️