Finally, after all these months I've identified the cause of the poor performance. Swirl flaps.
Replacement MAF and EGR resulted in misleading improvements
The symptoms started well over a year ago and resulted in me replacing the MAF and EGR valve. This misleadingly produced an initial improvements but the symptoms gradually came back and were backed up by the ECU light indicating an obscure P3263 fault. Diagnostics by the dealer and various technicians could only suggest an air flow problem which resulted in the MAF and EGR replacements.
Swirl flaps influence air flow as well
Of course there is one more component that can interfere with the air flow. The swirl flaps! Isn't hind sight a wonderful thing because it now seems such an obvious thing to check. That said I am and was a little apprehensive about the consequences of disconnecting components so this did deter me. When I did decide to disable the flaps it took mere seconds to pull off the vacuum pipe and block it off.
Fuel economy drastically improved to 60mpg on the motorway
The result was the ECU light immediately went out. The flat spots and lack of power disappeared and fuel consumption improved drastically. From a best of just over 50mpg with the faulty flaps to over 60mpg with them disconnected. That's better than I've ever achieved even when I bought the car at 3 years old. So have they been interfering all the time!?
To carry out further investigation of air flow vs RPM
I'm intrigued at the impact the swirl flaps have on performance so I hope to use my Torque app to carry out some more measurements of air flow versus engine rpm. I wonder if they will show a much cleaner correlation than the ones I took initially?
