|The fuel pump has a one way check valve in it that keeps the fuel from returning to the tank on the inlet side.
The fuel pressure regulator is what keeps the fuel pressure up, it is basically a dam that stops the fuel from returning to the tank until it hits the set fuel pressure.
If the fuel pressure drops when you turn the car off, but retains pressure when you block the return line, the fuel pressure regulator is bad.
If the fuel pressure drops when you turn the key off, but holds pressure when you block the inlet side feeding the rails from the fuel filter, then the check valve in the fuel pump is bad.
In order for the car to start, the fuel has to fill the lines back up and build fuel pressure for the injectors to provide the correct amount of fuel.
If you block off the return and feed lines and the fuel pressure still drops, that means either the injectors are leaking, or the fuel damper is leaking.
The fuel pressure regulator has a rubber diaphragm, about 10 years is all that is reasonable to expect out of a regulator and it should be changed as preventive maintenance.
Besides maintaining pressure, on a turbo car they increase pressure 1 lb for each lb of boost, when the diaphragm fails it can no longer increase fuel pressure with boost and will cause a lean condition under boost.
Since these cars are around 25 years old, they should be on there 3rd regulator at this age.
You can tell the age by the color of the regulator.
From 1990-1993 they were painted black when the cars were new.
In 1994 they came with gold regulators and the replacements were also gold.
In about 2005 they changed to Silver regulators, they are still currently silver.
So if you have a black or gold regulator, it should be changed due to the age.
The damper also only lasts about 10 years and also has a rubber diaphragm, the colors for age are the same as the regulator.
When a damper fails, it typically leaks fuel into the vacuum line, this causes the engine to run rough because it gets too much fuel. This is easily checked by removing the vacuum hose to the damper while the engine is running, when the damper is bad they leak fuel out of the fitting where the vacuum hose connects.
The Nissan parts are the best and last at least 10 years.
We offer them on our site, here are the links...
[ http://specialtyz.com/shop/default/300zx/fueling-ecu/300zx-fuel-dampener.html ]
[ http://specialtyz.com/shop/default/300zx/fueling-ecu/300zx-fuel-pressure-regulator.html ]