Key on, engine off 50 psi.
Engine idling 50 psi.
turn engine off and in 10 minutes it's at 41 PSI.

remove vacuum line while idling and it jumps right to 56 PSI. Rev the engine up to 2500 rpm and it holds 56 PSI. Turn engine off, 15 minutes later it's still holding 50 psi.

unless I'm missing something obvious, I think it's time for a genuine AC Delco fuel pressure regulator?

 

I just read some interesting info online. Remember mine is a 2003. Apparently in 2004 and above they went to a single 3/8 inch fuel supply line with no return line. Those are spec'd out to read 55 to 62 PSI constantly. 2003 and before had the 5/16 inch fuel return line with the pressure regulator. Apparently the proper way to test your pressure is to disconnect that fuel line. It's designed so with it idling it backs off the fuel pressure at idle during the high manifold vacuum. Wide open throttle the manifold vacuum drops and fuel pressure increases. To further complicate the matter, in some vehicles with the flex fuel option it retained the fuel pressure regulator, as well as the fuel return line and it ran a lower pressure rating on the regulator. I suspect that's why in many cases it says a Regulators compatible when in fact it is not in our vehicles. Probably the number one reason to use OEM to make sure you get the correct part number. If you leave the vacuum line disconnected and plugged, you are in effect reverting to the newer style where there's only the supply line and you get the steady 56 PSI fuel pressure. What I was reading cautioned that if you do that you will need to retune the ECU to recognize the different fuel rail system. It seems obvious from testing it as recommended with the vacuum line off effectively tests the fuel pump itself, which clearly regulates pressure itself without the fuel pressure regulator doing so off the fuel return. It would seem obvious with the incorrect lower pressure fuel pressure regulator installed, the fuel pump is constantly attempting to build up pressure and perhaps working harder than it should.. I wonder if that's what's behind so many fuel pumps not lasting as long as they should

 

These pumps will put up around 100 psi if dead headed, on systems that have no return line they have a PWM fuel pump system of a regulator mounted somewhere else other then the motor. Some where around 08 for the trucks is when they transitioned to the PWM controller, which is a lot easier on the fuel pumps.

Be very carful checking fuel pressure with the return disconnected and plugged, the pump will put up around 100 psi. with the correct equipment there is an SI explaining how to test the pump in this manor.

I usually get an adjustable regulator if I have a project that is pushing the limits of the injectors and bump the pressure up a little.

like I mentioned a little low on the fuel pressure is no big deal for the ECM and the injectors are size for around 80% duty cycle, plenty of room for adjustment.

Neal

 

The pump must have an internal regulator of some sort. Return line blocked off, pressure holds a steady 56 psi at all rpms. The pressure regulator on the injector rail has to be the issue. Releasing fuel to the return and keeping it at 50 psi max. (Which would be the correct regulator for the engines with flex fuel in 2004.....) My guess is somewhere along the line, previous owner or a shop installed the wrong part number. AC Delco with the correct part number arriving tomorrow.....we will see.