Just wondering if you have the fuel pressure intake vacuum reference line connected. Is it leak free?
Are you aware without this intake reference line, then fuel pressure is not indexed to the intake boost or vacuum?
For today's cars, the delta fuel pressure needs to be constant and part of the tune. Changing fuel pressure without adjusting the tune is a very bad idea. Why?
If the fuel pressure is raised and nothing else is changed, this creates a chronic rich condition. This is detected by the O2 sensors. The PCM responds by reducing injector pulse width (negative LTFT's). This is why it appears the adjustable regulator is not working.
Suggest setting the fuel pressure with the vacuum reference line disconnected to 40 PSI and leaving it.
Note, NEVER run a boosted application with the fuel pressure intake reference line disconnected. A lean condition will be the result (with the possibility of motor damage). Remove the reference line only for idle testing.
Consider when upgrading a return fuel system with a HUGE fuel pump, it's possible for the fuel pump to over run the regulator/return line. When this happens, the regulator will not be able to control the fuel pressure. This may be your issue.
This is clearly a case of "more is not better".
A FAQ on the issue can be found at:
http://aeromotiveinc.com/tech-help/frequently-asked-questions/faq-efi-regulators/
More information on how to select the "right" size fuel pump:
http://aeromotiveinc.com/2010/01/fuel-pumps-and-horsepower/
How to fix? The later MY Mustangs use a two speed fuel pump relay and ballast resister to slow the FP down at low power levels. An option is a fuel pump controller.
