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 GM spec on the fuel pressure WITHOUT the vacuum compensation line is 41-47psi, so if you unhooked the vacuum line and got 40psi, you're not that far off from the stock spec. Your losing about 4% flow compared to the correct 43.5psi, and the PCM can easily compensate for that with the long term fuel corrections.

The question is, what happens to the fuel pressure when you are driving, and the engine is under load. Do you have enough braided hose to allow you to pull the gauge back under the hood, and tape it to the windshield so you can see the pressure under load?

If you are sure the fuel pump is getting adequate voltage, the filter is not dirty and there are no leaks in the system, that leaves the fuel pressure regulator as a possible source of the problem. It could also be a problem with the line internal to the tank that connects the fuel pump to the support assembly. A pinhole there can cause problems with low pressure and fast bleedoff.

 

Well I blipped the throttle by hand under the hood and watched the guage increase in pressure but no I have not been able to see the gauge or tried to read pressure on the fly yet. Another thing I will be doing is to check voltage at the pump connection and buy a new fuel hose from sending unit to pump.