Getting nowhere….

My 2008 Q7 3.0 is causing a garage a headache and i am looking for any help. It wont warm up past 50-60 on motorway and when sitting idle goes to 90 and stays but as soon as driven off drops back to 50-60. Garage replaced thermostat the water pump and t piece one way valve and still doing it. They are now saying there is anothet aux water pump and if that is changed it will be all they can do. Has something been missed maybe coolant temp sensor or some googling says like the maf sensor. I just cant keep throwing money at this and it never gets done. They also said lots fuel around the injectors but not the seals and getting low fuel rail pressure codes but again they cant see any issue