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If you had 40psi pressure on the fuel rail and 30 inches of vacuum on the diaphragm and nothing out of the return you have either a stuck FPR or plugged up fuel rail or a bad ruptured diaphragm.
The FPR has a 40 pound spring holding it closed and the vacuum will pull against the spring to open it.
I appreciate the feedback. Most FPR discussion is generally if there is gas in the reference tube, change it. Since I hadn't ever run across anything else I thought I ask. I also didn't want to misinterpret results & throw money at a perfectly good part or tear one up trying to test.
So I go back & start raising pressure with no vacuum applied & I get to 46-47 PSI before the return starts to pass air. I cranked in some vacuum with my Mity Vac & I got an increase in flow when I got over 20 inches. I would have to guess that fluid pressure would be a more accurate test over compressed air, but it gives me something.
I also discovered at 60 PSI the fuel rail makes a great air horn. All it needs is a bell & some valves to change pitch. Next trip to the junkyard I'm picking up one for experiments.
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