Took the car out for a spin the other day with a few buddies. Me in my '10 XK accompanied by a GTR, Camero, Challenger and an authentic AC Cobra replica ...needless to say, it was a fast trip! ;-)

So we're haulin' *** on the back roads and we're all pushing the cars pretty hard ...but something seems odd to me.

I'm full throttle going down this straight stretch and we're not holding back. So I'm full throttle ...pedal to the metal ....6th gear. ...but when I lift off just slightly the car actually seems to like it wants to accelerate a bit. Lift off a bit more and it starts to slow down.

I noticed this before and had the dealer take a look at it but they said that the programing or coding for the throttle computer was fine. I'm not convinced.

You would expect that 100% throttle is at 100% pedal ...but in my case its as if 100% throttle is at 97% pedal ...as if by pushing the pedal to the floor I actually went past full throttle ...if that makes any sense.

Its like back in the carburetor days ...full throttle was when the butterfly valve was exactly at 90* to the barrel ...but if the throttle linkage was not rigged properly you could actually go beyond 90* and start closing off the barrel and loose power.

I hope I described that well.

Any thoughts??

 

I understand your description, well written.
I'm not a Jag tech, so I can only make generalizations. Off the top of my head some areas to look into:

1. The peddle position sensor (a variable resistor) attached to the gas peddle. As this is a throttle-by-wire system, the signaling resistor could have a flat spot, sending an incorrect signal to the ECU.
2. The throttle position (a variable resistor)sensor is out of adjustment.
3. The O2 sensors are not reading correctly (however this should throw a code.)
4. The MAP sensor is not reading correctly (this too should throw a code).
5. The throttle actuator is out of alignment. (as you describe, it could be over extending the butterfly valve in the intake.)

Of these, anything that has a variable resistor would be my first suspect, as they are inherent points of failure for cars throughout the years.


Hope others that know the Jag are of better help, my offering is just educated guess work generic to cars.


Vince

"Authentic Replica?"

 

I think your problem may be related to over-rich open loop fuel injection. Under "normal" driving, including light acceleration your Jaguar is in a closed-loop mode which keeps the air/ratio constant at 14.7:1.
When you go to wide open throttle, as you did, your auto's computer shifted to an open-loop mode. In the open loop mode modern autos dump just about as much fuel into the injectors as the fuel pump can deliver. At full power you will want to have an air/fuel ratio of about 12.8:1. Were you to measure your fuel ratio at wide open throttle you would probably see a fuel air ratio of 10-11:1, which is very rich.
One of the cheap tricks of adding power to a modern fuel injected auto is lean out the fuel mixture at wide open throttle. This does not affect emissions, which are only measured at closed loop.
You had the throttle wide open for a relatively sustained period. I know there are places in the UAR where you can drive this fast. It was running full rich. When you lifted the throttle slightly you may have momentarily "leaned" your fuel curve, adding power.

 

This is a classical example of restriction on the exhaust side of the engine. This restriction can be anywhere from badly designed combustion chambers, too small exhaust valves and exhaust valve opening or a too restrictive exhaust system. Every amount of air/fuel mixture you put in will be converted into exhaust gas. In this case, at full throttle the amount of exhaust gas is more than the engine can handle. The consequence is high back pressure, more pumping losses and intake charge pollution by still present and hot exhaust gases. So when you lift the throttle a bit, the amount of exhaust gas gets less. The exhaust side will be able to cope with this amount of exhaust gas and the result is more power with less throttle opening. This doesn't happen just with Jag engines, but also with a lot of others. There are cases in which the car had a higher top speed with less than 100% throttle opening or would accelerate.

Blown engines with turbo- or superchargers, are less bothered by this effect because the fresh air is forced into the engine. This will overcome the back pressure and will blow out any exhaust gases remaining in the combustion chamber.

A less restrictive exhaust system would be the first thing to do to solve it, although the restriction might be in the catalyc converters, in the exhaust manifold or in the engine itself.

It's a restriction "by design"

 

Thanks for the replies!

dutch07xk you say it's a "restriction "by design" ...does that mean it can be solved or that I'm gonna have to live with it.

Rey & CleverName ...thanks as well ...that being said, have you heard of this issue before or are these educated guesses?

 

You could try to fit an aftermarket exhaust system that is less restrictive. But if the restriction is elsewhere, it might not help at all.

 

As I'm not a Jag tech, I will always defer to those that actually know this car. My experience is from various cars of my past only.

I have both replaced and adjusted TPS sensors due to flat spots that were noticeable while operating the car. To me, it is a common failure item due to its location and environment.
A TPS sensor is easy to test with an ohm meter, as the indicator(meter needle) should swing smoothly from Closed to WOT (Wide Open Throttle). If the needle jumps about (indicating a dirty wiper arm inside the casing), then it is time to replace the unit.
When adjusting the sensor, you are given voltage values that correspond to WOT. This value is what the ECU expect to see in order to know exactly where the butterfly valve is positioned. If the TPS adjustment and voltage is off, then the ECU is making fuel calculations based on incorrect data.

My biggest problem is with the newer cars, most of these sensors are now monitored, so most stuff now just throws a code. I just don't know which components. or how far out a sensor has to be to trigger the CEL.

Please forgive... in many ways, my mechanic skills remain "old school" and I offer my thoughts only for helping you research a bigger picture.


Vince

 

Thanks again for the replies/suggestions

Rey: If this is what is happening, can it be fixed or adjusted?

CleverName: Of the sensors that you mentioned, are they all 'monitored' by the 'system'? Can they be tested individually or must I rely on a computer diagnostics machine to assess their functionality?
btw ...what is the CEL?

Thanks in advance!

 

CEL=Check Engine Light
Again, I have not worked on my Jag, so accessibility to something like the TPS on an XK is outside my knowledge range. To my annoyance, modern cars almost always require a computer tester any more. Jaguar (and many other brands) does not like giving owners enough information to maintain our own cars, as it hurts their bottom line at the dealership.

Again, I don't know how far out a maladjusted TPS has to be to get the ECU to throw a error, but feel it could be a possibility per your described concerns. It is monitored, as I see several codes that can come from it.

Sorry I cant be more helpful.
Vince

 

After a few frustrations with mine, we decided to take a new 2012 out for a spin, and it did the same thing!??!
...didnt have much to argue about when that happened
Would say I'm happy about it ...but I'm living with it.
Sorry for taking so long to reply, any updates from anyone else?