Low fuel pressure check engine light

My truck keeps throwing the low fuel pressure and low rail pressure ligth when I'm on the interstate cruising and stays are 10-12k rail pressure even when I step on it then the pressure jumps back to normal pressure and it's hit or miss also sum time I can go on a 600 mile round trip and it doesn't miss a beat and there times it throws the code no stop and I just installed a fass 150 a week ago the has a 2nd gen swap and ATD efi on it with about 211k miles

Is it P0087? It's hard to read your issue with no punctuation and a run on sentence like this one is cuz it just keeps going we don't know where it starts and ends.......

Are you saying it sticks around 12k even when you stomp on it sometimes? Could be a sticky FCA in the CP3. Usually they will act funny at an idle, also. You'll get injector rattle @ idle from high pressure.

Sorry I was at work trying to type between customers . The truck keeps throwing low rail pressure and fuel pressure check engine ligth only when I'm on the interstate . Also won't go over 10-12k rail pressure once the light comes on

Could be lots of things.
Lift pump, filter, cp3, FCA, pressure relief valve, connector tubes loose, injector issue


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I installed a fass150 10 days ago and the code came back one time and now I can't get it to come back on

I got it check engine light to come on first code was P0087 cleaned it then 30-45 mins later P1011