I have a 92 Z28 TPI. My fuel pressure is extremly erratic while the car is running. So erratic that the needle on the gauge is just a blur in a span from about 20 to 60 psi. It also seems to be related to RPM the higher the RPM the faster the needle almost as if seeing the injector pulses, but at WOT (and only at WOT) under load and at high RPM the span narrows to 40 to 50 psi and the needle slows down.
If you check the pressure by switching the key to the on position (car not running) so the fuel pump comes on; the pressure snaps to 48 psi and remains constant.
The car has an adjustable FPR. Changing the adjustment only moves the span up or down while running. The pressure adjusts at a constant rate and is stable when the car is not running using the method above.
Anyone got any ideas were to start looking?

 

Scat,

If the pressure is rock-steady with the fuel pump running with the engine off, that just about eliminates the pump check valves as a problem.

You may have some unusual wear on the FPR disc and seat valve parts. You might also have a spring in the FPR that is getting harmonic with engine vibrations. Or you may have a length of hose between the Schrader valve and pressure tester which is getting affected by engine vibration.

Are you checking with a dash gauge or hand held instrument? The dash gauge might be subjected to some other vibrations as well, causing some of the needle fluctuation.

------------------
Later,
Vader
------------------
"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0

 

Vader I have a FPG that is in the car and uses an isolator that is mounted directly to the fuel rail. I have also used a hand held gauge. I got the same result both times.

 

Could it be the fuel filter??