I'll start by saying that I'm new to all this.

I have an 8.3 ISC Cummins with a high psi CAPS fuel pump. There is a priming fuel pump that turns on when the key is turned to the ON position, and it only runs for 30-60 seconds and turns off. The mechanical side of the caps fuel pump is then primed and sucks fuel from the tank.

I had problems with it starting, could hear gurgling sounds from the priming pump as if was grabbing air. Found three little mounting bolts on it and got about two complete turns on each bolt. The gurgling air sound disappeared and it started.

Preemptively, I decided to try and save the CAPS fuel pump by installing a fass system. The fass pump provides 16psi of fuel to the caps at all times. The model of fass I got has a return line, always cleaning the fuel of air and particulates. Instead of running the return line all the way to the tank, I tapped into the engine return line. It started and ran great, until I pressed the throttle, then it died. All the injectors are getting fuel, the air is bled out. I loosened the banjo fitting from the engine return line, and its shooting fuel out of it when the engine isn't running. Would that psi prevent the engine from running? I thought there was a check valve installed, but even then, if the return line is pressurized from the new fass return, would that pressure be my problem?

For troubleshooting I was going to disconnect the return of the fass pump and stick it in a five gallon diesel can, then try and start it with no pressure on the engine return line. Does anybody have some advice for a newbie here? Am I on the right track? I did a little research before installing, and a mechanic who installs fass systems said I could plumb into the return line like that. He also said he mostly only installs the model of fass that doesn't have a tank return.

Thanks for any advice