Finally I bit the bullet and took in a brand new and insanely expensive air-flow meter. It was a Mercedes part built by Bosch in September 2010, and a last one in stock. Stephen helped me with the installation. I also replace the air-inlet temperature sensor (measuring the air-density or air-mass, whatever that may be) which is optional and supposed to affect the air-fuel mixture in a slight way so that the mixture would not be too thick. The air temp sensor cost me nearly nothing compared to the new air-flow meter.
The whole engine and drive-train felt completely different after putting in the new air-flow meter. It is now much more torquey and responsive, and is more wiling to kick down and revvs from 4000rpm and onwards. The idling is now much quieter and even more smoother. The engine sound is healthy. I can finalize say that the engine is 85% to 90% of what it used to be when it was brand new. Although it is a personal speculation, but I am more than happy to feel and say it.
My rationale was that the fuel consumption had been high and I hope the new air-flow meter would correct it. And it feels like it does even though I cannot prove. I have changed every aspect of the engine components and it would be a stupid not to change the most expensive and yet crucial one. Before today, it was a big gamble and I am glad the lottery was won.
