Out of my own curiosity, what happens when the FPR takes a crap?

Does it fail in such a way that the pressure is unregulated (i.e. it's always at either min or max?)

Does it fail in such a way that it "tries" to regulate the pressure but fails to meet the requested target?

In listening to my truck (I'm 99% sure my FPR is failing), it sounds to me like it has too much pressure giving it the "lopy idle" sensation. Additionally at light / part throttle and empty cruise I am noticing more timing rattle which *if* I'm getting too much pressure would make sense because my injection event is happening to rapidly. Whereas when I "get on it" all the extra timing noise goes away as I'm using more pressure. Granted this is a SWAG as my logging data of a stock tune is gone due to a hard disk swap ;)

If your FPR has taken a dump or is about to, one of two things is going to tell you that it is. One, your won't have a steady idle and you'll notice it in gear more so. Stopped at a light and all of a sudden the truck will surge and want to hit the car in front of you. Fuel pressure spikes, injectors throw extra fuel, kind of scary. Two, I'm not sure if EFI would show it the same as the tech II but if you look at it on a Tech II it looks like a crack addicted three year old high on sugar drew on your wall with a marker. Theres no pattern to it whatsoever. A normal fuel pressure regulator should have a pattern similar to that of a heartbeat. Up and down and repeat continuously but in a regular and predictable pattern.

When mine went, I saw and felt both of those symptoms.