Re: 2.4 JTD power surge/smoke puff...
Hi, I have had a succession of turbo diesels and live in the twilight zone of existence in Aberdeen where darkness falls early and light comes late, the opposite in summer. I own a 156 JTD 16v Mjet 150 and it is no different to any of the others during the hours of darkness, a blue grey puff out the rear as the foot goes down, especially noticable at night with a following car.
All of us want the best mpg and tend to cruise at 'fouling up' speeds, your mechanic is right, give it a blast at higher revs with some diesel treatment in the tank and forget the pennies for the sake of a good clear out every now and again. This will save you cash in the long run avoiding repair costs for complete siezure of the EGR valve.
Previous cars, Golf TDI, VW Passat TDI, Xantia HDI, current Mazda Bongo 2.5 Turbo, and the Alfa - by far the best.
I have have also noticed this with numerous diesel hire cars on business.
It is really down to the nature of the beast, everyone fails to realise the potential of modern diesels, but they do like a hard drive now and again to 'clear the tubes'.
Hope this reassures you a little.