Overall there are so many confusing observations to make. To begin with I filled up at the beginning of the journey with the higher grade diesel as had been recommended by the BMW dealership. They advised that this was a good idea every fourth tank or so. (Presumably it contains less impurities and perhaps some cleaning additives). But as I've noticed before the performance problems can seem more acute after a fill up?!! However, I'm reluctant to get side tracked down fuel related routes.
So after a journey returning 50+mpg at 70mph I once again connected my ELM interface and collected data over an urban route with my smartphone. This was after the hesitation and faltering seemed to be getting particularly annoying.
I then disconnected the vacuum pipe to the EGR valve to see if it was in any way related to this device. It did reduce the symptoms but not completely and performance seemed flatter but was preferable to before. For urban driving the mpg was certainly in the 40's so nothing dire was happening to the fuel consumption. It seems that the EGR valve is more of an emissions related device and so I decided to keep it disconnected for the return leg up the motorway and collect data once again.
Having looked at the graphed data it is radically different to that collected previously under no load! There is very little in the way of a correlation between MAF, boost etc vs engine revs as there was before. Whether that is to be expected or not is unknown at this point.
One thing I did think that must have a correlation if boost pressure vs air flow rate and this is shown below with the EGR valve disconnected i.e. permanently closed. I'm intrigued that above a certain pressure the flow rate becomes erratic. An air leak maybe?
This is with the EGR connected during the urban driving. There are obviously a number of differences such as considerably lower flow rate for a given boost pressure which would be expected given that the EGR valve is letting in exhaust gas. But there is also no sign of the valve shutting and the flow increasing either.
Another exhaustive search of the internet revealed many more BMW 320D owners with similar problems and spending a fortune with specialist but no magic bullet!
Next step check for air leaks after the turbo.
