Ok so here is the scoop. In June of 2013 my Mustang started to hesitate on the road when I would press on the gas (similar to what happens when your fuel pump starts to go). I replaced the fuel filter, the fuel rail pressure regulator, and also flushed the fuel system. It also was starting hard and has continued to start hard both cold and warm. I replaced the fuel pump in September 2012.

Fast forward to Oct 2014. After not hesitating at all for many months the car is now doing it again and is triggering the p0191 code. It starts hard when engine is cold and hot, but sometimes it does not start hard at all . It does not hesitate when I press the gas gently, but if I push the pedal down just to give it a little more throttle (not flooring it) it sometimes hesitates. The car idles normal and the fuel pump shuts off after a second when I turn the key on as it is supposed to. I am CONFUSED. Could it be the pump or the fuel rail pressure regulator? Both were replaced with OEM parts from Ford.

 

Have you changed the fuel filter and cleaned the TB? When my TB was dirty it would hesitate.

 

I've done all of that including cleaning the MAF sensor. No dice still

 

P0191 Ford - Fuel Rail Pressure Sensor Circuit Range/Performance


Low or no fuel
- High fuel pressure
- Low fuel pressure
- Faulty Rail Pressure (FRP) Sensor
- FRP harness is open or shorted
- FRP circuit poor electrical connection
- Faulty Fuel Pump
- Vacuum leaks


I would check the sensor and all the wiring as well as all of the above.