Van came if for service.m.o.t.,Engine oil leak,MIL light on and int.lack of power.Oil leak was turbo oil seal failure(no impeller damage)replaced turbo,washed out intercooler and h.p. pipework.Fault codes logged on SP ACR4 G2 with descriptions(no cross ref. to AD,or OBDII library)1826,pre-heater plug circuit,permanent, heater plugs faulty or in open circuit 1808,coherance between cam sensor/engine speed, intermittent no engine speed signalor speed interface.remove crank sensor(cracked on head)fitted new sensor,code 1808 cleared.When engine R.P.M 2,750-3,250 stationary,misfire/hesitation occurs(and starts hunting).Pedal switch sweep is gradual smooth reading.checked maf sensor.248v @ idle,3.25v @ 2k,3.5v @3k, peaks @ 3.85 volt.put new dealer maf on. now peaks @ 3.74v.have checked wiring across g/box and head for chaffing.by passed fuel filter(no air)Checked leak back,all good.Rail pressure goes to 1100-1200bar when fault occurs.Drops to 730 when clears.Fitted doner fuel pressure soleniod.Any ideas,suggestions greatly recieved.