Fuel regulator location

On that vehicle it is part of the fuel pump/sending unit, the image below is what it looks like. But the code you list should not apply to your vehicle because the regulator is not electrically controlled and that is what that code means, that the ECM cannot control the regulator because of a circuit failure. Being that your vehicle does not even have that circuit I would say it is a false code.

An easy way to test the regulator is to connect a fuel pressure test gauge to the port on the fuel rail. You should see 55-60 psi with the engine running at an idle. Then shut off the engine and watch the pressure it should drop less than 10 psi in one minute.

Other than the code what problem/issue are you having?

I put a fuel purge vent valve top of the motor, and a fuel filter.

Cadillac dealer said to follow the fuel rail to find the fuel pressure regulator.

The fuel pressure regulator attaches to the fuel return pipe on the fuel sender assembly. The fuel pressure regulator is a diaphragm-operated relief valve. A software bias compensates the injector on-time because the fuel pressure regulator is not referenced to manifold vacuum. The injector pulse width varies with the signal from the mass air flow (MAF)/intake air temperature (IAT) sensor. With the engine running at idle, the system fuel pressure at the pressure test connection should be between 380-410 kPa (55-60 psi). With the system pressurized and the pump OFF the pressure should stabilize and hold. If the pressure regulator supplies a fuel pressure which is too low or too high, a driveability condition will result.

Well I just do not know what I am doing.

Nah, you just need to decide what to do. As I said that code should not apply to your vehicle. If it is running okay and no other issues, clear the code and see if it comes back. You could replace every part of the system in response to a ghost code and still not "repair" it because the ECU would not see the repair as being completed.
Would not be the first time a ghost code showed up.

Thank you I will do that.

Thank you I will do that

It is having a hard time idling and it runs on like three cylinders.

Okay, are you sure the code was not a P0301? That would indicate a misfire on cylinder number one.

Low fuel pressure could cause that but a common issue on that engine has to do with the timing chains failing and causing those same symptoms. If that is what happened you are looking at major repairs.

Can you make a video and load it on this site? Hearing it as it starts and runs might throw a light on the problem.

Okay thank you.

To see if that is the issue you could do a compression test. That would tell the tale quickly. I would expect many more codes as well.

That is a great idea Steve. I will be busy tomorrow.