2008 Corolla 1.4d4d Inadequate Power Problem At Higher RPMs

[theengineer]

Hi All,

I have a 2008 Manual 1.4d4d Class Corolla at 149.000 kms in Turkey (It is something like that: E140 Photo). I love it so much but I have a trouble with my car. None of the car mechanics can solve the problem yet, somehow . I have spent a lot of money than I expected. Although I want to sell it, I am still spending because I don't want the next user to be dissapointed with Corolla. 

My nuisance problem is that the car decreasing the power at higher rpms, especially when climbing hills or overtaking. It is especially occuring at 3rd and 4th gear between 2000 and 3000 rpms. In order to keep the car at the effective power I must gear it down, otherwise I can't overtake or climb.  At those conditions the motor doesn't produce constant power to pull the car. The power randomly oscillates. The oscillation is not so clear, because, except the driver, only a coutious people at the front seat can understand the oscillation. But everyone in the car can sense the inadequate power of the car. The problem has continued for 2 years and for 15000 kms.

The first think that comes to mind is Turbo. Turbo performance is checked by repairmens, but its performance and pressure are seemed very good. Air quality sensor, clutch, diesel filter, air filter and engine-oil are also changed with genuine parts. Fuel tank and EGR parts are also cleaned. The selenoid valve that starts/controls the turbo also changed with new one. I am also using high-quality diesel.

Could you please inform me about what I should do in order to solve this problem?

Any help will be appreciated. Thanks in advance.