those pressures are VERY low for a 4.9. You should see approx 55 PSI key on engine off and it should maintain that pressure within 5 PSI for at least one minute after you shut the key off. There could be a regulator problem or a problem with the fuel pump in the front tank or both.
Pull the vacuum line from your regulator, is it dry or is there gas present?
I talked to a guy at the part store and he said if the pump is working and the engine runs, it is most likley the fuel tank selector valve. He said fuel pumps usally either work ,or they don't. Said this is a very common problem on these Fords and most people who replace the $150+ pumps usally come back for the valve anyway.
The good news is that one tank functions perfectly, so it's not the regulator or a leaking injector. Since the front pump module holds pressure, even though it is low, it's not the check valve in the pump module, another common problem. The failure to build enough pressure has me stumped though. I need to run through the step-by-steps in the PCED manual, I'll get back to you if I find anything on testing the selector valve or another likely problem.
OK I checked through the manuals. As nearly as I can tell, Ford discontinued the selector valve on or before the 96 model year. The selector switch on the instrument panel doesn't position a valve, it selects which of the 2 in- tank fuel pump modules is operational.
Following the sysptoms you gave me, it seems very likely that the problem is related to the front tank module, but it may not be the module. Check the supply voltage to the front pump. It is supposed to be 10.5 volts or gearter. If it is not then you need to trace the wiring to find the problem. If it is 10.5 or greater, the problem is likely to be the fuel pump. If you can get your hands on a 96 PCED (Powertrain & Emissions Control Diagnosis Manual) look in section HC for the step-by-step procedure for diagnosis
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