the car is a 96 z28 a4. car stopped in a parking lot and didnt fire for a day, checked it the next day and it fired right up for a min or two then died and wouldnt start again, after checking the spark at the plug i put on a fuel pressure gauge and it showed 42-45lbs when the pump was turned on then the second the pump cut out it dropped to 20 for a second then 0, could this be a fuel pump? regulator? injectors?

 

All three. You can check the regulator for a ruptured diaphragm with a vacuum pump (like a mighty vac) or check it for a gas smell in the vacuum side, the injectors you can check by pulling the whole thing up and seeing if they leak, but it's probably that the check valve in your pump has failed or is stuck open.

 

it doesnt hold pressure long enough for me to think its the injectors, unless the're REALLY leaking haha. the relay sounds like its working good, would the car not start at all with a bad regulator or just bog down and idle rough?

 

Regulators have a spring in them holding them partially closed and a vacuum diaphragm to overcome the spring to open them more and lower pressure when demand is low [high manifold vacuum - (throttle blades closed)], so if the diaphragm failed the pressure should still be there and some extra gas might go into the intake anyhow, but it would probably take a catastrophic failure of the regulator to have it pass all the fuel back to the gas tank. My vote is on the fuel pump check valve, besides it's a common failure mode. And I told you how to test the regulator.

 

i just bought a whole pump assembly since i was getting in there anyway, holds pressure just fine now , there had to be a failure of the check valve because the pump works just fine.