hello everyone i have a bit of a problem i would like some input on. I have a 1995 Z28 M6 completely stock other than the removal of the smog pump. i have an autometer fuel pressure gauge installed underhood attached to the shrader valve via a steel braided fuel hose for easy access to the gauge. ive noticed the fuel pressure at around 25-30 psi and a noticeable stumble real bad at times at part throttle. At that point i checked the FPR by unpluging it and got an increase of 10 lbs on the fuel gauge so 30 lbs with vacuum and 40 without. at that point i replaced the fuel filter and removed the fuel pump and replaced the pump with a supposed good used pump from a friends impala SS that he replaced for an upgraded aftermarket type. After i installed the pump and fuel filter i primed the pump without starting car and got no more than 30 lbs on the underhood fuel pressure gauge. turned it on took it for a test drive and noticed that its still the same with a stumble at part throttle and at full throttle it will backfire and at times idle drops to shut off unless i give it gas. Ohh forgot to mention I have also replaced the optispark as well about a month ago. Now it could be that this used pump i replaced could have been bad but i removed it from the impala myself reading 42 lbs two weeks ago. i also noticed after i turn ignition off fuel pressure drops within 3 minutes.

 

The fuel pressure dropping that quick points to a leak in the fuel injection system, and the fuel pressure seems to be on the low side. Look for a bad fuel pump, bad pressure regulator, leaky injectors. The fuel pressure gauge shold be arond 40 PSI with the engine at WOT under full load. The test you done were under a no load condition which can give false results. The pump may be able to provide proper pressure at no load, but can't deliver the flow needed to maintain pressure under load.

 

and if you havent do so yet, look at number 33 here 4th Gen LT1 F-Body Tech Aids