Slowly loosing fuel pressure

Hi, I have a FASS 150/150 that is about a year old. Over the past few months, my fuel pressure has went from a rock solid 14psi. to 12psi. that will get sucked down to 10 or lower on a WOT run. I changed the fuel filter a week ago, but that changed nothing. I haven't made any other changed to the truck except to send my rail pressure gauge off to be fixed. What should I check, or do to fix this?
Thanks in advance, Colin

Ideas anybody?

14PSI at idle seems low to me. My 150/150 is at 20/21psi at idle and never drops below 18psi wot. Poss some kind of restriction in the pickup not letting the pump draw enough. Maybe the pressure regulator spring inside the fass is getting weak and it is bypassing more fuel than it is supposed to. Try pulling the spring and swap it with one out of another fass that is pushing the proper pressure.

Joe

Springs are almost never the issue with pressure.

I would first check your gauge.....as that is the most common issue we see related to pressure. Do you have an isolator between your gauge and engine? I would start there if you do.

Second would be restricion. Might try the bucket test and see what that gives you for results. Just put the FASS Suction and return line into a bucket and see if the pressure is any better or not.

Yes on the Isolator, I bled the line to check for air bubbles pre-isolator, and post isolator. I made sure the coolant runs from the isolator to the gauge no air bubbles, no leaks any where.

cbtoyz said:

Yes on the Isolator, I bled the line to check for air bubbles pre-isolator, and post isolator. I made sure the coolant runs from the isolator to the gauge no air bubbles, no leaks any where.

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Welp....can you temporarily bypass around the isolator for testing purposes? It might not be an air leak as much of a sticking diaphragm. I would push it if you didn’t have the exact symptoms of a leaking/malfunctioning isolator……