From OAP...EX Auto sparks now retired 80 years.... fully conversent with Bosch K & KE system.....with what you have replaced so far , ignition must be good, Fuel should be good with what you say.....i feel you should check what is coming out of the 4 injectors.....may be some "Crud...debrie"..has been drawn up from the High pressure pump, bear in mind the system runs 85 / 90 psi all the time....ive wrote before how to check injectors....but in case you have not read....all 4 injectors must be lifted out of manifold with there pipes connected.....place injectors in 4 small bottles ...or what you have suitable to contain petrol...remove the Top of metering unit you will see the metering Flap....now unplug the Fuel pump relay...on the relay plug you will see wires Red black pin 87....and Red pin 30....bridge these 2 with short piece Wire, Pump will now Run all the time..OK..so far...now NO Fuel will be coming from any injector...very slowly push down. the metering Flap...just a little...you will see the start of Fuel comeing from each injector....pushing more the pressure goes up...check you have good some flow pressure to all 4...may be you wont in your case...as you say one plug very lean , any debre that gets through to the metering unit...and pipes will cause a blockage. you must see a nice flow of pressure at injectors. hope of help, there is a way of cleaning any pipe you feel blocked. will advise. once you read this. Peter