I have been chasing a low fuel pressure problem for a while, I added a FASS 150 about 6 months ago and did little to help the problem. Finally with a help of a friend we diagnosed the problem to a CP3/Regulator problem, I finally got around replacing both the CP3 and fuel regulator today an now an interesting thing happens.

With the FASS on the actual rail preasure stays much higher that desire, but it is perfect with the FASS off.

Any ideas about this???

In order for the FASS air separator/filter system to work does the FASS pump needs to be running?

Update on this, after looking at the pressure readings a bit closer, what is actually happening is that after a WOT situation the actual takes longer to get down to desire with the FASS on that with it off, but it does gets down to the correct level.

Are you running a FPRV that is a "race Valve" that does not bleed off high pressure? That could cause it.

Also what fuel pressure are you running the FASS pump at-- It only needs about 8 lbs.

Higher than about 9.5psi at inlet to the CP3 will give you that. Common problem since the early days of the FASS/AirDog pumps.

Hmmm, I don't remember adjusting the fuel pressure for the FASS, I just installed the thing, is there an adjustment or do I need a fuel pressure regulator.

Is there a prefer place to install a pressure gage to test the inlet pressure to the pump?.

The FPRV is factory.

You can tap a pressure gauge in at the OEM filter housing using the plastic bleed screw threads, and a adapter from that converts from metric over to pipe thread.

The FRPV should not be active in normal use. The idea behind the shim mod is to push the activation pressure high enough that it does not activate.

Too much lift psi will cause what you are seeing. You do not need much psi at all. It's all about volume and keeping a gentle push on the inlet of the pump at all times.

The FRPV should not be active in normal use. The idea behind the shim mod is to push the activation pressure high enough that it does not activate.

Too much lift psi will cause what you are seeing. You do not need much psi at all. It's all about volume and keeping a gentle push on the inlet of the pump at all times.


That was my understanding, and I was under the assumption that the FASS or any other lift pump is calibrated from the factory to provide volume with not much pressure. I will call FASS customer support to see what they say.

One question, is running the truck like this a problem, is it going to cause something to fail?? I have the option of running with the FASS off, should I?

So long as pressure follows somewhat closely during normal operation you should be fine. When you chop the pedal some carry over is normal, but if this carry over spikes too high it will beat up on the safety valve.