danalex

Added some new 'maxtow' gauges. I know some folks don't like 'em, but I don't think that's my issue...

Fuel pressure:
As long as I keep it under 65MPH, the psi reads 14-15 psi.
As soon as I top 65mph, the fuel gauge begins to act wildly...flopping from as high as 22psi to as low as 6-7 psi.

BACKGROUND
The truck has died twice in the last 4 months.
First diagnosed as a bad fuel pump (Raptor 150). I believe the diagnosis was true; this is my 3rd Raptor in 6yrs; lifetime warrantee, so replaced it.... Computer showed no codes, and there was zero psi at the VP44. ((VP has 30K miles on it))...
Second time, I was @ 70mph on the highway and watching the maxtow gauge wildly flopping...until it dropped to 8, 7, 5, 0 psi and --truck died. Diagnosed this time as AIR in the line. My mech bled the lines and truck comes to life!
((both previous times, there were NO CODES)).

Question: What is happening at +65MPH??

KATOOM

I think you need to isolate the gauge or sender as the possible culprit. The only way to do that is to hook up a fuel pressure test gauge. And if you installed JIC fittings all the way and removed the fuel filter housing then you'll have to be creative and plumb a 1/8" MPT port in the fuel line and install one of the OEM schrader valve test fittings.