Smoke from MY2001 XJR

[CecilVonSol]

Hi, wonder if anyone can help.

History
I have an MY2001 XJR, bought in December 07, currently with 73k, I have had the timing chains, tensioners, guides etc. replaced, inspection showed excessive play on driver’s side. I got the newer water pump fitted and a new pressure cap and thermostat at the same time, as a precaution. I also had the gearbox fluid and filter changed as well as new front disks and pads. The diff input seal was shot, so the input shaft was replaced, I believe that the seal is not available separately.
I still have to get the rear disks and pads done and am pretty sure I am up for some suspension bushes to be done soon also.
The car uses no oil whatsoever, it has not moved a bit on the dipstick since December and about 4000 miles. I have checked after it has been parked up for a while and it leaves no sign of any dropping fluid.

I never really pushed the car hard, until I had the work done above, it drives like a dream, apart from one small thing that has now happened twice.

Problem
Both times the same scenario, outside lane of the motorway, 70-80mph, if I try rapid acceleration (WOT), car changes down a gear, supercharger gets loud, car begins to really motor, smoke appears out of the back, quite a bit !, no loss of power, smoke goes away fairly quickly. If I try the same but from slower speeds there is no kickdown on the box, no smoke whatsoever and then I run out of road.

Is this possibly un burnt fuel getting dumped out into the exhaust when the car kicks down to 4th gear and the revs spin up, or is it something more sinister. No smoke at any other time and it does not use oil, uses a heck of a lot of fuel tho

Many thanks in advance

Col

[CecilVonSol]

I had a good look around what I can see of the engine today and the only thing of note is that the cap that should cover the pipe where the dipstick for the auto box should be is missing.
Is it possible that on kick down, at speed there was a surge of auto box fluid up this tube? Is it okay to drive the car, at low revs with this cap missing and has anyone priced up how much the auto box dipstick on the XJR is?