I posted this query on a UK site and it didn't lead to any definitive answers so I'll open it up to the vast knowledge over the Pond -

I have previously owned an X40 and then an X300 - both 6 cyl cars fuel injected. They felt like damp squids when I bought an X308 V8 4.0L 2002 yr. That was about 6 years back. I've driven the X308 for nearly 1/4 million miles on business (with only the TB failing), but in all that time I always noticed that it lacked 'urge'! By that I mean I could waggle the throttle pedal a good 1/2" whilst driving at any speed and it made no difference.

Thinking the 2002 car was getting tired (and due to fail), I bought a yr 2000 Sovereign V8 4.0L with 60k on the clock. Immediately I noticed that any movement of the throttle pedal results in a jump in speed. That thing just wants to rocket away. I assumed that the 2002 car was just tired/worn.

I then bought a yr 2000 XK8 4.0L (3 months younger than the Sovereign). It too lacks 'urge' exactly the same as the 2002 car! It's so frustrating - the big Sovereign goes off like a scalded cat, and the 'sports job' trundles along!

The advice at the time was - take any slack out of the throttle cable. Funnily enough, the only car with noticeable slack was the Sovereign! Next was - clean the MAF. Did that to no effect. So I swapped the MAF's over (Sov to XK8). They had different part numbers (by a couple of digits), but were otherwise identical. No change!

I have done 'hard resets' and then retrained the gearboxes - no change. So I'm looking for ideas. Does the ECU have a programmable EPROM in it. Can they be safely switched over? Could it be the PPS or TPS on the throttle body (an expensive swap), or the electronics that operate the throttle plate?

 

I would suspect clogged catalytic converters on any higher mileage car. They tend to clog up well under 100k miles, especially if the car is not driven hard on a regular basis.