my new fuel pump

umm....it's flowing way more than your stock pump LOL....stay off the gas pedal dude!

When you changed the pump, did you also change the filler neck grommet? Usually once they're disturbed they leak, so you might not be experiencing worse economy, you might be losing gas...

Texstang9682 said:

So I just recently put in a holley 255 fuel. My fuel mileage has drasticly gone down since. Whats the deal with this? My car is a 96 with the return style fuel system so this whole thing befuddles me.

I am getting the car tuned next week , maybe that will help. I have checked the plugs and they look like theyre not running too rich.

Any experts care to elaborate on this?

Thanks in advance for any help, CLINT

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What's the fuel pressure at the rails?

If the additional pump volume is, for whatever reason, overwhelming the FPR and return lines, pressure will back up in the rails and the result will be more fuel being used. Do you have any SES/MIL indication of an overly rich condition?

Although I wouldn't expect a 255lph pump to cause fuel to back up in an otherwise stock return system perhaps there's another fault (e.g. tank venting?) that's come to light because of it. First thing I'd do is check the rail pressure both at idle and under driving conditions (acceleration, cruise, coast etc...)

Texstang9682 said:

I replaced all gromets including the filler neck one on the tank. No leaks whatsoever. I havent checked my fuel pressure regulator recently. Last time I checked it it was at around 40 psi at idle. Now the new pump is the forced induction kind and will operate at a higher pressure. Maybe I need to get the gauge and check it to see if it went up some more. The forced induction pump is for anticipation of getting a blower.
By the way what should my fuel pressure be at right now? With and with out the vacume line attached. If you need to know what I have on my car its all the bolt ons minus the PI heads.

Thanks

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Keep in mind that the fuel pressure is determined by the fuel pressure regulator (FPR), not the pump. The pump needs to have a high enough capacity to deliver the required volume at that pressure (that is, the injectors are trying to empty the fuel rails and the pump needs to keep up: if it can't the pressure in the rails will drop...) but otherwise it's the FPR and return lines that determine pressure.

40PSI actually sounds at the high end of the range for idle. The system attempts to maintain a pressure drop across the injectors of roughly 39PSI. If you're seeing 40PSI when there's a manifold vacuum of 15-20inHg, I'd almost think your vacuum line is not connected to the FPR or the FPR is defective. Or, as is possible, the FPR return aperture is not large enough for the 255lph flow from the pump and fuel is simply backing up behind it, increasing pressure in the rails as a result.

What does the fuel pressure do when you drive the car? Cruise? WOT?