It seems the most common failure mode of the FPR is for the internal diaphragm to rupture. This allows unmetered fuel into the intake via the vacuum line attached to the FPR, which will cause the engine to run rich, possibly to the point of not running at all.
Another failure mode is for the FPR to get stuck open, or open too easily, which causes the fuel pressure to run low and the engine to run lean. If the fuel pressure is low enough, the engine won't even run.
Final failure mode is for the FPR to stick closed or open with too much difficulty, which causes the fuel pressure to be high and the engine runs rich.
Diagnosing fuel delivery problems isn't difficult. 1st, run the engine (or try to start, if a no-start), then pull the vacuum line to the FPR and see if it has gas in it. If yes, then you have a bad FPR. If no, then hook up a fuel pressure gauge to the fuel rail and see what the fuel pressure is and diagnose from there.
