I have dived into the world of fixing the vac leaks on my 2008 XJR. Turns out the hose to the Secondary air system had fallen off, as well as the Y piece that goes to the vac tank and the SC bypass, as well as the brake booster O ring. It now seems to have all of the little leaks fixed.
The issue I have now is on WOT it is very hesitant and shudders and struggles to gain any speed past 80~ but on half throttle will comfortably do more. It seems to start having issues around 3500RPM and onces it gets towards 5000 it makes a clattering noise.
Any ideas where to start looking?

Thanks

 

At high revs / load, it's not going to be an air/vac leak.

Could be fuel supply (change filter, check pumps work fully) or such as blocked cat(s).

In case MIL flashes, could be failing coil(s).

Check for any pending codes - any hints can be useful.

 

I'll do the fuel, I was focused on vacs because of the bad state that was in when I got it. No codes, other than a lean code on bank 1 that only showed up on a warm start if it was driven slowly, and that seems to have cured with the now working vac system.
Cheers

 

I would check fuel pressure. Has the filter been changed?
Since you have an "R" it runs both fuel pumps from 3500 rpm and up. So your hitting a problem right at 3500 rpm. I suspect your running out of fuel but this is a mystery too because it should set a code for incorrect fuel pressure?
But my bet is on fuel pump.
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I have ordered the filter, hopefully it turns up soon, and I will get the OBD tool out and go for a trip down the local dual carriageway.

 

Your car might not have the Schrader fitting? We did find that Jaguar dropped it at some point on the 4.2L V-8.
I used a mechanical pressure gauge hooked to the fuel rail and ran the hose so I could drive around and look at the fuel pressure.
BUT I was chasing a fuel pressure regulator code so different than what problem you have.
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I have replaced the fuel filter, and the old one was in need of replacement, but the issue has not been fixed. I managed to get it to return some codes, which are the random/ multiple misfire and various cyclinders (all even) so it seems limited to one side of the engine.
The one thing i have noticed, is that the commanded AFR was down at 8 at one point where as the measured was up at 14