I put a new 150gph Fass Platinum on my truck a few weeks ago. I've noticed that when I start the truck in the am, and the outside temp is about 5C outside, I've got about 15psi fuel pressure. When I start the truck at the end of the day at work, and the outside temp is 15-20C, the pressure is about 12-13psi. When driving when warm out, it will drop to about 10psi under fairly hard throttle. Doesn't seem to matter if the tank is 1/2 full or full. Any ideas what would cause this?

 

Could be the diaphragm in the fuel pressure sending unit. Try putting a shop light under the hood, pointing at the sending unit, over night with a 45w bulb on it. That will keep the sending unit warm. If the pressure is what it is when it is warmer, not a big deal. If it is the same the problem is not with the sending unit and may be somewhere else in the fuel system.

Easy test though.

I had sender issues with mine and had to put a second snubber on it. Low pressure readings. It was in the winter so it was always cold. I have an 18" line from the VP to 2 snubbers then my sending unit. After I added the second snubber, everything worked just fine. That was about 15k - 20k miles ago.

 

The only time I have seen fuel psi drop from temp was when I was on the interstate in over 100*F (38*C) and had less than 1/3 of a tank. I dropped about 1-2 psi.

 

Thanks guys. I'll try and warm up the diaphram, don't have a 45 watt bulb in my trouble light, and I need to be careful I don't start a fire. I'm a little suspicious of the electric fuel pressure gauge, when I put the 150gph FASS in I went for a 10 mile test drive. The gauge was bouncing like crazy, ie hammering from the pressure. I put in a needle valve, which cured that, but it's entirely possible I did some damage to the gauge, although I was hopeing that 10 miles wouldn't have been long enough. Anyone know what gets wrecked on an electric gauge from hammering, the gauge itself or the sending unit (diaphram)?

 

Could be the temperature induced viscosity change. Well, okay, it is.

 

No clue if the VP44 has a overflow valve, so disregard if it doesn't

But I have had the same symptom on my truck and it ended up being a weak overflow vavle. I would have 18 psi when the engine was cold and it would drop to 14 psi when warm.

 

Summer temps create lower fuel pressure due to higher fuel temperature!
Thinner fuel. Every summer I notice start up pressure is 1-3 psi lower on my FASS 150 than was in the winter.

MY Isspro gauge was bouncing badly for months before installing a needle valve. This was 4 yrs ago and it still works well.... so no worries mate.
Every winter I have to tighten down the needle valve a bit (almost fully closed off) and every summer I can open it 1/4 turn.... also from difference in the the fuel density due to OAT.

RJ

 

I think so too.
Even with the mechanical pumps on my 12 valves I've seen about a 2 psi difference between 100° and sub freezing temps.

 

Thanks guys, greatly appreciated. I'll keep you updated, pulling the 5th wheel this weekend, so that will be a good test.

 

Your 2 psi drop under heavy throttle also sounds normal. Particularly if you're towing heavy.
Falling much below 10 psi would worry me a bit. If it does drop lower, I would start to worry about my VP-44.

I have an 18 psi spring in my 150 gph FASS. Might want to consider that.
I never get below 15 psi....even when I was at 600 hp and racing at the track!

RJ