Hello everyone, I am having a sort of similar issue with my 2.5 DSE auto recently. The top speed seems almost limited to 70 MPH and the speed drops steadily to about 57 MPH on long steep hills. Before the problem started, I was able to accelerate uphill without an issue. So far I have changed the oil and filter, air and fuel filter. I've changed the MAP sensor and checked the MAP pipe is clean, with no cracks and is securely fitted. I found during investigation that the turbo hoses were a bit soft and replaced those. The acceleration is now really good and the engine revs smoothly up to 3000 RPM ish when pushed, before changing up. However the problem persists on the motorway and I'm still stuck at about 72 MPH at about 2500 RPM. I don't normally drive faster than 65 MPH, but like to have a little in reserve which is now lacking.
At some point a tuning box was installed, which used to flash when the engine was running, but no longer does so. I'm wondering if this might be the cause, although it seems odd that the performance is there at lower speeds.
I read elswhere that changing the fuel pump relay might help and I plan to do that tomorrow.
Has anyone had a similar experience? Any advice greatly appreciated.