Another fuel pressure test is the leak down test. To perform this test, hook your gauge up and take your readings while the engine is running. Then shut the engine off and observe the pressure reading. It should hold steady for at least 30 minutes. It will drop a slight amount when you first shut the engine off, but it should hold steady after that. If it doesn’t and you see a steady drop in pressure, you have a leak somewhere.
One of the most common leaks is the check valve inside the fuel pump. Each electric fuel pump contains a check valve designed to hold pressure in the system after shutoff for a faster start-up later. One of the symptoms of a bad check valve is a long cranking time when you first start the engine after it's been sitting for some time. The reason for this is that the fuel pump needs to fill the entire system with enough pressure for the engine to run before the engine will start. Here is a video showing what's inside a fuel pump.
The check valve isn't the only thing that can bleed off fuel pressure. You could also have a leaking fuel injector or bad FPR, as we talked about earlier. To separate the two problems, simply perform the same test above, but this time pinch off the return line and feed line going back to the tank when you shut the engine off. This will isolate the fuel rail from the rest of the system. If the leak goes away and you don't see a pressure drop, your problem is in the tank and is likely the check valve in the fuel pump. If the problem is still there, then you know the issue is somewhere in the fuel rail, which could be a leaking injector of FPR problem.
So how do you find the leaking injector? I often find them with the power balance test listed above. A power balance test can isolate a problem cylinder. With a leaking fuel injector, I often see a wet spark plug on the cylinder where there's a fuel leak. It might also have lots of soot caked on it from running rich. Sometimes you can remove the fuel rail and observe the injectors after you pressurize the fuel rail. You need to be careful with this, as fuel injectors are often just held in by the fuel rail. Once you remove the fuel rail, the injectors aren't secure and can pop out if pressure is applied behind them. Only do this test if the injectors are held onto the fuel rail in some way. You might do this with a piece of wire or something. You don't need to start the engine with the fuel rail removed; in fact, you can't. All you need to do is turn the key to the ON position for three seconds or so to pressurize the fuel system. For the most part, I think a wet or sooty spark plug that sticks out from the rest is all the info you need.