My 2010 Mercedes S550 V8 with 221K miles runs rich on both banks LTFT reading are at negative 19, Dealer wanted to change the Intake and the o2 sensors on both bank and quoted me for four grand, my private mechanic said that they are fine, and the problem is with the fuel rail reading. Is it the cause? Please help
Why not start with cleaning or replacing the 12 year old injectors, and a new fuel filter?
Watch Scotty's video, "How to Make Your Car Run Better with Spray Cleaner" and do as he shows. If you've never cleaned the MAF sensor, it's had plenty of time and mileage to build up gunk.
Look at your MAF sensor data, if the Coyote V8 has one. Some cars use MAP (Manifold Air Pressure) to figure out the same info, my 2017 V6 Mustang has a MAP sensor, my girlfriend's EcoBoost of the same year has a MAF. It should read roughly the same displacement as the engine. If it's too high, definitely try to clean it.
If all of that checks out, look at how the mixture is when a friend is driving on the highway. If it's less rich, the injectors are probably dirty/ worn out and dribble gas at idle. A few rounds of fuel system cleaner may help clean them up.
Thanks, I will start that