Hi all, I'm looking for a brainstorming session to see if we can come up with ideas for where my car's fault might be, so I can focus my mechanic in next time it goes to the garage.

Since I bought the car a couple of years ago, I have encountered an infrequent intermittent fault where the car will enter restricted performance mode, but only while accelerating at high speeds. Usually, this happens between 120mph and 140mph, but I have just had it happen around 105mph more recently.

- There are no error codes my little reader can see
- There is no issue with maxing the revs or power from a launch or from medium speeds. The engine gains and loses revs freely. I can be quite aggressive with gaining and losing speed at country road speeds for an hour with no issues at all.
- I don't feel like the car is low on power most of the time at lower speeds.
- The warning goes away after 15, 20 mins of sitting still with the engine off.
- I've asked a Jaguar specialist to look into it, but they found no issues, and it's understandable they couldn't really test it because it only happens at speeds they cannot reach on public roads.
- I can't hear any odd noises from the engine
- There may be a link to the gearbox, as it does seem to happen after as it revs high
- I have seen it happen in Drive and in Sport, but have never tried it with the paddle gears activated.

Current theories:
- Extra air somehow getting into the system causing lean fuel mix
- Air intake leak around the intercooler leading to excessive temperature of the incoming air to the engine. Apparently there is no code for "air too hot" on this car, despite the sensor being there and there being a code when the engine is used in Range Rovers.
- Problem with intercooler pump leading to excessive temps again.

Are there holes in my theories? Which do the experts here think most likely? What have I missed? Is there anything I can say to my mechanic to help them find the issue?

I can think of two different scenarios for your problem...
A small and/or intermittent vacuum leak. The best way to find one is to have it Smoke Checked.
Second is a partially clogged catalytic converter (or both).

Either one of these conditions could cause weird sometimes problems like you describe.
Codes for these don't always last, as it's difficult to read codes at 140 mph.

Be good to use this thread as a learning experience too. Could you describe how each of those faults might work, and why they might lead to this behavior?

A vacuum leak makes stuff way too lean, and a cloggish cat makes stuff way too rich.
Too much airflow, then too little airflow.
Maybe check the STFTs and the LTFTs via OBD2. They SHOULD all four be near zero. You'd mostly have to watch (or graph) the trims while under load and high giddyup.

I had precisely the same issue on the xjr… Coincidentally, as CeeJay suggested, it was partially clogged cats.
Replaced the pair and all the symptoms went away.

Not even pending codes? (Some tools don't report them, sadly.)

I had somewhat similar behaviour, but I've a different engine, and it was IAT2 (if you have one).

Typically this occurred on long trips. The engine light would come on. then go off.
Historical codes were thrown implying O2 sensors. In fact, there was one visit to the shop where the O2 sensors were replaced due to the codes… but that ended up not being the root cause. The actual cats were.
If I recall correctly that was at about 80K kms.

An FYI, I’m on my 3rd set of cats in the xkr. Still haven’t had any negative episodes following the cat klean solution CeeJay suggested just a few months ago. I will try cat klean, if needed, again. But if I don’t have immediate success, I will change them up.

My practical test was simple: following the cat klean, Reset the ecu, fill with 94 octane, perform the standard “learning” run, then give her the italian tune up she wants.
If a code is thrown, she gets new cats.
If not, I win.

For some reason though, the rear tires are wearing much, much faster than the fronts.

Ouch! That seems very rare. There may be (probably is?) some underlying cause worth finding & fixing.

What code reader are you using? Not all code readers are equal and can read JLR codes resulting in no codes showing. My speculative guess would be a failing supercharger/auxilary coolant pump.

lol, the underlying issue is 160,000 kms and 2 excellent pre race nights in Montreal where there’s lots of engine revving to red line and to the point where the cats glow red.
What’s that smell, lol.

Show us what's under your hood and maybe we might can point out and notice something broke,missing,or out of place to help you fix it

Loads of great advice here, thanks all. It's got to go in for MOT soon so I'll ask them to also check for vacuum issues, triple check the cats, and check the intercooler pump specifically. Hopefully they find the issue!

As an aside, I usually run 95 octane fuel (UK). This holiday I ran it on 99 octane Shell rip-off fuel, and despite some deliberate late-night abuse, I couldn't replicate the issue. That would lean towards the whole "too much air, lean mix" issue, right? Cos the engine is unintelligent, it doesn't use the extra octane, so what would be lean at 95 would be less-lean at 99, all else equal, right?

Doesn't the '99 octane Shell rip-off fuel' have less ethanol, E5 v E10?

It does, yes... Unsure how that would interact here, would have to look up the relative combustion principles.

I can tell you that these issues occurred on the E5 95 octane before the UK shifted mainline fuels to E10.

The States have had 10% alcohol for decades. Maybe we're used to having more problems so they seem...... normal now?

An air leak which only causes trouble at high speed when accelerating sounds like a unicorn.

Hunt for something else

May help, may not, and also I have a 2008 4.2 SC XKR, so again not sure how relevant this will be but I got a Restricted Performance message just tonight omw home while taking the long way home and some spirited highway blasts at some mph. Message clears after just a couple minutes, but now afraid I hurt it, I baby it home. Hooked up my icarsoft and after some digging thru data streams, noticed manifold pressure was low. Popped hood and sure enough, heard the unmistakable hiss of a vacuum leak. Found a rubber elbow had popped off. Popped it back on, idle smoothed, manifold pressure now back to normal. Will drive it tomorrow and hopefully confirm fix.

I don't know that this is necessarily all that uncommon. Especially in the case of a pressurized engine the manifold system changes from vacuum conditions to boosted conditions depending on engine speed and load. Various check valves are used to regulate vacuum distribution to ensure that components that should only see vacuum are not exposed to high pressures. Sometimes these check valves fail in ways that only show up under boost (or pressure testing). And the leaking under boost can cause reduced performance and/or check engine lights.

I will acknowledge that I am not very familiar with Jaguar engines so this is just conjecture as applied to Jaguars. But I do also have a 996 generation Porsche 911 Turbo that has a maze of vacuum operated functions, and those forums are full of discussions on failure modes that only occur under the same conditions as discussed in this thread - i.e. under high boost. (And yes of course turbos are different from superchargers, etc. etc., but the failures are not necessarily related to the way in which the pressurization is achieved.)

Certainly not impossible but exceedingly rare on these engines.

Interesting, but nothing like what the OP has.

BTW a leak you can hear is huge LOL