I've fought this for years now thinking I've had it fixed for a while then it comes back. Even with the stock pump (it was an oreilly stock replacement) I had a 5 psi drop in pressure after say 1+ hours run time. Put a walbro 255 in (yes I know too much pump, I know that now but it works) and thought all was well but the drop came back with a vengence last fall. Thought the stock wiring just couldn't handle the load of the walbro so I ran 12 gauge power and ground directly to the pump using a relay and the stock fuel pump power wire as a switch to put fused but otherwise direct battery power to the fuel pump. Again thought it was fixed but driving back and fourth to school I am getting a lean condition after about 1+ hours of driving. I now have an external vacuum referenced regulator and at idle I get 30 psi reguardless of run time, but cold I get 50 psi with the vacuum hose off, and after 1+ hours I get 40 psi with the hose off. So what gives? Two fuel pumps do the exact same thing. I've tried everything to fix this and I'm clueless. What could it be?

 

Think we've got it figured out. Its either the regulator which I HIGHLY doubt it is because its done this both on the stock regulator and with the external unit, or the EVAP system isn't working (no surprise there) and the fuel tank isn't venting and slowly working its way up to a vacuum causing me to lose pressure. Now all I've gotta do is confirm this.

 

interesting, glad you figured it out...

 

One way to confirm if that what its doing, would be to take the gas cap off and see if it changes.

 

I did that and there was no change. I checkd it cold and it had 30 with vacuum and 40 without and I swear it had 30 and 50 when I originally set it where it was. Drove it a while and got the pressure drop to show up. Gauge read 25 and 35. I eliminated the pump by turning up the pressure north of 60 psi when it was idling at 25 psi and the pump didn't hesitate one bit. The regulator seems to be working just fine. Can liquid filled gauges be effected by heat? Apparently they can. Seems like the fuel pressure is just fine but the gauge is wacky.

 

I was going to ask if you were using a liquid filled gauge mounted under the hood. Yes, they are affected by heat. The heat causes the liquid to expand and put pressure on the borden tube. Thus showing a lower then true pressure reading.

RBob.

 

Maybe I should drain some of the liquid off so it stays more accurate. I had one gauge go bad and leak the liquid into the gas I guess because it went from liquid filled to nothing. I broke a couple 15 pound gauges from running too much pressure on them too. I have bad luck with gauges. Finally I just got this 0-100 psi gauge and it works fine other then the drop. At least now I know what it is.

 

someone at the track told me that opening the cap wont do anything because it is in the bottom of the tank under all of the fuel. if you look underneath at the little white thing above the rear end you can take that off and see if you can suck through it. that is what controls the venting.

 

Well I don't know that its connected but my fuel pump died on my yesturday. It still runs and will pump about 25 psi at idle but off idle it goes to nothing. It had been screaming for months and the last week or two its been dying out at heavy throttle but I just assumed it was tune related. Just got back from oreillys to get a Delphi vortec 350 pump and harness, strainer, filter, the works. 136 dollars worth but I wanted the best pump I can get so thats what I got. My stocker made it 155k and was still working fine, it just couldn't push the pressure I needed and the lines were nearly rusted out.