Stop baiting the newbie
brianbrianyi, yes the engine lacks in just about most ways that we are used to driving, the gap between 3rd n 4th is immense, going up a hill in 4th is not going to work unless you're in a 20mph speed limit area, drop to third and live with it mate, the engine is somewhat something you will get used too.
Motorway you're looking at 50-65 max without screaming the arse off it.
By all means give the engine a check over, no harm in it, but from what you're saying, it's normal behavior for a 2.5TD engine.