I recently had issues with the sending unit, as you all know the fuel gauge gets all wacky and does what it wants. I tried to save money by only replacing the sending unit. Once I did that the sending unit was working fine but I was now immediately losing fuel pressure after key off. Then, if the car sat for 15-20 I would have to key up the car a few times before starting it or crank it a couple times before it would actually start...Since I just had the fuel pump out, and it took a certain amount of "finesse" to get it out, I figured it may have finally seen it'* day. Plus it is the original pump and has like 140,000 miles on it. Before buying a pump I checked the FPR and didn't see any gas coming out of the vacuum line/port. I checked the dipstick and it didn't smell like gas either. The fuel filter on this is MAYBE 6 months old. So i went ahead and put a pump in. Voila, no more losing pressure. Fired up no prob. Waited 20 min and it fired right up again. Waited 40 more minutes and it fired right up again...dun dun dun...20 minutes after that it had to crank for a couple seconds before firing up...30 minutes after that the same thing happened. Then I let it sit for an hour and it fired right up...Fast forward to this morning...the car had been sitting about 8 hours. I made sure it got nice and cold. Came out this morning and she fired right up. Threw the gauge back on it and she was holding pressure fine. It idles at about 48 psi. Key off and pressure stays steady around 50 psi. When I remove the vacuum line from the FPR pressure jumps up to around 53-55ish as one would expect. All the vacuum lines appear to be fine. I don't see any crack or anything like that. So why when I throttle it does it drop to 42 psi fuel pressure? There'* no way it should idle at 48 psi and throttle at 42 psi. I'm thinking it would have to be the regulator unless the brand new pump is somehow bad. Any ideas?

I also forgot to add that after the key is off and it is sitting for about 10-20 min the pressure dropsto around 42ish and holds steady. This seemed pretty normal to me.

I just ran to the store and grabbed a fuel filter. I'm gonna toss it on in a minute. While driving the car seemed to drive just like it always did. Everything seemed normal. I'm seriously confused.

I swapped out the fuel filter and it'* still acting the same. I sprayed around all vacuum lines with starting fluid and nothing happened so I'm fairly certain there is no vacuum leak. Once again, it appears to driving fine. It doesn't feel sluggish or anything even tho fuel pressure shows up at 42 psi when throttled and 48psi when idle. The only piece of the puzzle I haven't swapped is the FPR. Now I don't know if it'* even worth the gamble since it seems to be running fine.

I've seen the same thing on my 97 Bonneville. I don't know why it does it either.

So should that be considered normal? it just doesn't make any sense haha

I think what you are seeing is normal operation. Opening the throttle relieves pressure in the fuel system since it is allowing fuel to flow, rather than sit and build pressure.

Yea, what Roy said!

That totally makes sense when you put it that way. I was just under the impression more fuel more pressure...this leads me to my next question. I'm idling at 48 psi and throttling at 42 psi but my buddy told me fuel pressure should be 53-59psi (I'm not sure if he referenced Mitchell or Alldata but it was one of them). Could those numbers he gave me be wrong or could i have a pump for a non supercharged motor?...my original fuel pump idled at 48psi as well but i just replaced it because the check valve went bad.

Can anyone verify what the correct fuel pressure should be at idle then at throttle?