possibly an injector stuck wide open, may not even flag a fault. i know from my younger years playing with 306 turbo diesels it was a case of no smoke no poke lol after turning boost and pump up! we used to get them to smoke like fuck on a straight road and turn them down little by little till the smoke went ha ha. my zx turbo diesel used to nail my mates rs turbo till 3rd gear with a nice cloud of unburnt skeg lol