The fuel system in my Beretta is pretty typical. The fuel pump is electric and is located inside the gas tank. It sends fuel to the filter near the gas tank and then to the fuel rails on the engine. The injectors are mounted on the fuel rails. A fuel pressure regulator that is vacuum controlled returns unneeded fuel back to the gas tank. When the engine is idling or under low load, a lot of the fuel sent to the engine compartment is directed back to the gas tank. When fuel demands are higher and then engine is consuming more fuel, less is returned to the gas tank. The main advantage to this design is that extra fuel is instantly available to meet increased fuel demand. I also believe a constant flow fuel pump is less complex and more reliable.
Fuel filters are designed to flow in one direction; I've heard installing one backwards will result in a lack of fuel pressure.
When I change a fuel filter, I like to cut it open to look for junk. However, they can be full of goo that restricts the fuel flow and still look ok. So if there's any question, I think it's best to just replace it. Be sure to follow the safety instructions found in the service manual; the fuel system can be under pressure even when the engine is off.
The main symptom of a bad fuel pump relay is that it takes a lot of cranking for the engine to fire. The ECM normally engages the fuel pump for a few seconds when you turn the ignition on to pressurize the fuel rails. If you don't hear the fuel pump run for a few seconds when you turn the ignition on, you should suspect there is a problem with the fuel pump relay.
The manual says you should see 40.5 to 47 PSI when you turn the car on (without starting it). You should hear the fuel pump come on for a few seconds and then shut off. When you start the car, it's supposed to drop 3-10 lbs. Mine started at 40 and dropped to 37. I also hooked the MityVac to the fuel pressure regular and convinced myself that it was working. (No vacuum boosted the pressure to about 45 and other vacuum values produced numbers between there and 37 lbs.)
If you decide your injectors are bad, there is apparently an alternative to just buying new ones. I've heard you can have your injectors rebuilt at many performance shops. This sounds attractive since the rebuild includes testing.
I did run one bottle of injector cleaner through the car. I'm sure Richard Petty and STP appreciate my purchase but I was unable to detect any improvement. This might be because there is already injector cleaner in name brand gas. And as I've said, I believe the domestic injector design to better.
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