I have high mileage 2012 MKT. It has developed a strange problem. When engine reaches at 41 mpg car start to stutter or knock and it does that until 50 mpg. Before and after that it run smooth with no problems.
If i press the gas hard then it crosses that 40-50 and runs just fine.
I am not sure what is causing this.
Anyone faced that kind of problem let me know what you did to fix it.