It goes like a train lowdown but when it gets to about 4000rpm it just goes flat and hesitant and just will not rev at all . it does not make a difference what boost its at,and its not misfiring at all.Has any one got any ideas as its driving me mad.thanks terry.

 

sounds like it is in limp mode m8 , maybe try asking one of the tuners

 

Have l6 ecu got limp mode?.