2014 MERCEDES-BENZ CLS63 Engine Problems

the car started showing a low coolant message around 2017 about twice a year. I felt it was too often the message would come up. I would too off coolant each time to address it without seeing any leaks ever under the car. then 2018 thru 2019 the message would appear about 3 or 4.times per year. had it checked by Mercedes service and they advised nothing was leaking from the car and found no codes indicating urgency in repair to address. my car started failing to turnover after cranking and in July 2021 it failed to turnover at all ever again. took it to the factory sego r dealer and they examined the car for repair from 7/2021 to 12/2021 when they gave it back to me. after getting me to authorize $8,000 in repair cost to get it to turnover, and must needless repair, they advised it needs...