I just removed the tst and mp-8 off mytruck and now with teh NEW stage 3 pump I am throwing a P0088 code and the truck sounds like a cold 12 v I have checked the plugs on the rail and the fca and they are making good contact and if I unplug the fca no change in idle sound. I have been reading and the group is saying the fca maybe bad or the rail sensor. how to tell which one is bad. truck starts fine just high rail pressure at idle Rob

 

It sounds like a contaminated relief valve on the fuel rail, especially if it started after you swapped the pump.