renault master 2.5tdci oil problem

Hi.This is my first posting.I have a 03 master 2.5 td which was running great but using a lot of oil.I was advised that the turbo was worn out although it did,nt seem to smoke much.I fitted a second hand unit which was fine until it went onto over run.I fitted a brand new unit did 300 miles and the same thing happened.sent turbo back but they have returned it to me having given it a clean bill of health.What else can cause over run? I cleaned out the pipes and inter cooler.Can you do a compression test through the glow plug holes?.

Any help would be most appreciated as we are planning a 3 week trip to Europe an am losing confidence in the van