It's a new pump and pressure regulator, but it will go from 48 to 54 psi running. Anything under 50 runs bad. i am a mechanic , can't figure it out.
My car has 200035 miles.
My car has an automatic transmission.
One possibility is an internal leak in the system. The leak could occur at an injector or in the tank. Check the fuel pressure with the car off and the key on. Cycle the key a few times to build pressure if you need to. It should not drop more than a few pounds in five minutes with the motor and key off. If it rapidly looses pressure, use a pair of long nose pliers and find a rubber fuel line you can pinch to isolate the fuel tank from the fuel rail. If you pinch off between the tank and the gauge, and it holds pressure, the leak is in the tank. If you pinch off between the gauge and the fuel, and the gauge holds pressure, the leak is in the fuel rail or fuel injector rather.
It is possible the new pump or regulator could be the issue as well. Definitely check for a leak in the system before you decide this though. The pressure are low according to factory specifications. GM vehicles are very sensitive to fuel pressure for sure.
I would also check that you have a good source of power and ground at the pump. I have seen low voltage due to bad connections numerous times. Fuel pump connectors and the connector that is midway down the frame rail will often develop corrosion.
If you should need further assistance, have one of our mobile technicians come to you and help diagnose the issue firsthand. The description of this inspection may not be exact to your symptom, but the diagnosis is the same.