XK8 Restricted performance message and system Too Lean

[Dave Eynon]

Hi, - on behalf of another member with a 2000 4.0 XK8
I am receiving fault codes P0171 System too lean (Bank1), P0174 System too lean (Bank 2) and Restricted Performance message.
I have had the car's fault codes read and reset by a garage several times and so far the actions taken include: -
Inspection for air leaks and brake cleaner sprayed around ducting etc with no increase in rpm observed.
Both O2 sensors replaced.
MAF and throttle body cleaned.
MAF sensor swapped with a secondhand unit from Autoreserve.
The spark plugs were replaced.

The Restricted Performance illuminates after several miles, but the cars performance remains the same.
Can anyone point us to the most likeliest cause? If it is an air leak what/where else should we check? Have other owners suffered similar messages and fault codes and identified the cause? Why does it always occur after several miles of driving usually on dual carriageway roads?

[Astromorg]

The brake cleaner method may work if you're lucky, but I suggest a more positive approach would be to do a smoke test that blows smoke out of the smallest cracks and crevices. If you're handy, there's even how to build a DIY smoker on You Tube

Two pipes not to miss are those of the crankcase breather system - one from each cam cover, as they can be rather brittle and run under covers out of sight in places.

[Dave Eynon]

Hi, I agree with spending more time on proving there are no cracks or air ingress. Build your own smoke machine - I will look into that!
Cheers
Dave