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JimT

Just reread the thread again. Before going too far down the rabbit hole you should verify there's no restriction in fuel flow coming from the tanks. Even a partial blockage there could be starving the pump as it call for more fuel, rail pressure drops off, truck shuts down, pressure equalizes and it starts back up.

Unless you have some evidence that debris got passed the primary and secondary filters I wouldn't think that it made it into the rail. Even after the filters there is a mesh screen on the fuel line banjo bolt coming into the gear pump. A screen which should have be inspected when the pump was replaced.

Since you just mentioned the excessive rail pressure loss, on top of the recent replacement of the gear pump and priming pump, lets focus on that for a second. The pressure relief valve shouldn't be opening anywhere near 8700 psi. It should be somewhere above 40,000 psi. That's not the only way fuel flows back though. The injectors could be leaking into the fuel return line or the quell tubes going to the injectors could be leaking into the same return. Those tubes are crush fit to the injector so if they are removed they need to either mate back up to the same injector or be replaced. They need to be torqued down properly too. You could also have a leak at any connection point along the fuel rail itself. Go through and retorque everything to spec.

Finally, if there's no restriction in supply and if the primary and secondary filters are not running dry then that leaves everything from the point the fuel exits the secondary filter to the engine for a leak. You could still be sucking in air somewhere before the gear pump. Or there could be a leak in the high pressure common rail circuit. Really the only thing to do at that point is to methodically go through the diagnostic tests outlined in quickserve to pinpoint the cause. Guessing can only go so far.

Rawze

A red engine is not a blue or yellow, or black engine. Never assume that it is similar!.

Also, fuel rail and injector leaks are often into the return line system, and not directly into the cylinders ... but injectors can leak into the cylinders too.

Here is what you should be doing as far as testing the fuel rail for holding pressure properly ... AND LEAK TESTS SHOULD BE DONE AT 30,000 PSI, NOT JUST SIMPLY AT IDLE PRESSURES!!!.

There is a proper set of procedures for it and a video on how to do it here...
http://rawze.com/forums/showthread.php?t...2#pid69262