Help: Instantly Dropping Fuel Pressure

[NoMoreMustangs]

I have a problem that is quite perplexing to me, perhaps someone can aid me in finding the problem and hopefully a solution.

The Problem
With gauge on fuel rail (with schrader core removed and with it in), fuel pressure is 39 PSI running at operating temperature and as soon as the key is turned off, the pressure drops within 1-2 seconds to 0. I have verified it was not just the gauge by removing it and seeing no pressure right out of the valve. The car starts and idles well (I cannot road test currently). The fuel relay is bypassed (as the ECM will not run the pump with the key on otherwise).

I cannot verify if this problem was present before replacing the following:
Fuel Pump with Walbro and Hotwire Kit (http://www.kirbanperformance.com/product/230/HIGH-PERFORMANCE+IN-TANK+REPLACEMENT+FUEL+PUMP+KIT+#1540.html)
Stock Injectors with TT 42 lbs with matching chip (https://www.turbotweakstore.com/ProductDetails.asp?ProductCode=1017)
Fuel Tank (http://www.gbodyparts.com/product_i...d=726&osCsid=9873ebe366f865b06ff992800e2e0bbb)
Adjustable Fuel Regulator (http://www.kirbanperformance.com/product/224/Billet+Aluminum+ADJUSTABLE+FUEL+PRESSURE+REGULATOR+#6547.html)
PCV Valve (http://www.kirbanperformance.com/product/43791/AFTERMARKET+PCV+VALVE+#7276.html)
Fuel Filter http://www.kirbanperformance.com/product/222/AC+Delco+GAS+FILTER+#6540.html

What I've tried
I've followed the fuel lines from injector to tank and verified no external leaks are present. I've used this resource - http://www.gnttype.org/techarea/fuelsystem/fpsys_check.html - to complete steps 1, 3, 4 and 5 with each step yielding the same result of fuel pressure dropping to 0 as soon as the key is turned off. With the outlet line of the fuel regulator blocked, the pressure on the gauge rapidly increases too 100+ PSI so I would say the pump is putting out enough pressure (Although with no lines blocked it never exceeds 39 PSI). These instructions say that the problem must be the injectors, however I just replaced them, the pump, the wiring, the regulator and the fuel filter. I put the old regulator back on to test and still got 39 PSI and instant drop.

Backround Information
When I bought the car it started and drove well, (although it had a clicking under acceleration, which I attribute to speedo cable? Although advice on that would be appreciated as well). The place I stored it for the winter in started it and moved it a number of times and it ended up running out of gas (I kept it low because I planned on changing pump). With a full tank it would not start, pump wouldn't run and I ended up bypassing the relay to make it run, although it still would not start. I ended up replacing the above list of parts and with that completed, it fired right up but upon checking the pressure, well that brings me to here. The car does have a small intake leak into the plenum, although this should not affect this problem.

Thanks in advance. I've added a poll, because who doesn't love a good poll?

 

[bpman1234]

I know you said the fpr was new . But when this happened to me my diaphragm was leaking in the regulator or you might have stuck injectors leaky o rings at injectors

 

[pacecarta]

leaking fuel line in the tank..
or
bad (damaged) check valve at the pump from over tightening the clamp (its at the tip of the pump outlet)

 

[kjizzle]

I second the check valve. It's not an uncommon issue, and it sounds more plausible because of the issue being carried across two different regulators.

Nice write-up, but the way. Very comprehensive.

 

[Lil Truck]

I expect it is the hose and/or clamps on the fuel pump. If you have to replace them get good quality fuel injection clamps that will not cut into the house and the proper in tank rated fuel injection hose. !!! Other hoses will soften up and blow out taking your head gaskets out and possibly your engine!

 

[NoMoreMustangs]

I've determined it to be the fuel pump check valve, as another test where I clamped the fuel inlet and outlet lines after the pump has run showed that there was no pressure drop across the injectors. That coupled with switching of regulators only leaves the fuel pump check valve. As the car still starts easy and runs well, I'm just going to leave it as is until I feel the urge to change it. Appreciate the responses.