2016 Ram Eco Diesel low fuel pressure
 

Trying to help a friend out with a 2016 Ram Eco Diesel. vehicle will run idle for about 30 second then stalls . Switch it on an off several times crank over then starts but stalls again after 30 seconds. If you start it up and raise throttle it will run but is limited to around 3000 rpm. let go of throttle and press it again and it stumbles then run to 3000 rpm again. Has code p0087 low fuel pressure. Now here is some extra . The vehicle only has 11,600 miles on it. He want take to dealer because he has deleted exhaust system and changed to a performance computer, Also running red farm fuel and has ran it for thousands of miles without changing fuel filter. He has installed a new fuel filter now but still has the same problem. Any information would be greatly appreciated.