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Indeed, FPRs don't always fail as expected, as I also found out.
In my case, the cold engine would start fine but would run rich, splutter and almost die for the first 30 seconds or so. After that, it would run and idle just fine, every time, until it cooled down again.
Slightly black spark plugs predicted some failure developing LONG before error codes and CEL were thrown. Long as in years before. Only near the end did the spluttering start.
So I did the usual checks like smelling for fuel on the vacuum side of the FPR and sticking the fuel return line in a bucket to see a healthy flow. All looked fine. I did not have a fuel pressure meter.
The only clue I had was the already mentioned warming up time of about 30 s. And when I realized that was the time it took the O2 sensor to start giving useful data, switching the ECU from warm-up-program to the normal running program, I decided to replace the FPR anyway.
It turned out to be the right decision.
The injection timing during the warm up is based mostly on ECT, and completely without the feedback of the 02 sensor. This needs for the fuel pressure to be in the ball park. Slightly high, and the mix also becomes slightly higher than intended.
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