I have the same OM642 engine in a 2007 Vito ... owned from new, full dealer service history, completely stock. Started to run into problems last year at about 96k miles ... had it in limp mode (sometimes with EML on, sometimes not) no less than eight times and off the road for almost 7 weeks in total. Biggest issue was that the exhaust manifolds contain liners where they meet the block, which are known to de-laminate and send debris into the turbo. New manifolds and new turbo required - engine out job, and a bill of almost £4k (after a hefty discount). Also had the EGR valve fail last year, and the MAS, and a DPF temperature sensor (which had seized and had to be drilled out). And (very minor) a couple of glowplugs. Prior to that the CPS failed, and the DPF pressure sensor. Touch wood it's going OK now but after a total repair spend of £6k in the last 12 months (not to mention the major inconvenience) I wouldn't personally consider trying to increase the power output of an engine that's already done 101k miles. I also wouldn't buy another modern/complex diesel, if i could possibly avoid it!
