2005 Vauxhall Combo - Major engine problem

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hi everyone i've got a big problem with my sons combo diesel van, firstly
it wouldn't start and was very smokey, so our local garage put it on the computer and it said egr valve and air mass meter so we changed them, then we changed the crank sensor, now it won't rev over 1500 rpm and the top end has a knocking noise previously the engine was sweet as a nut, so back on the computer and the forums so we had the timing checked as i had read of the timing chain problems, however everything all pinned into position so that was ruled out, we've had the turbo out now and thats ok as well, has anyone got any ideas as what this could be as we and the garage are now really stuck and my son is demented as the vans been off road for 4 weeks today!! any thoughts would be gratefully accepted and thanks for reading my whine regards andy