What could cause fluctuating fuel pressure?

[team39763]

Yesterday my fuel pressure was 6psi, like it has been since I set it a month ago.  But I went out there today and it was 7, then when I started the truck it went to 8psi.  Then I turned it off and ran the pump with the motor off and it went to 9psi.  I flipped it on a few more times and it would bounce around between 7 and 9.  I tried setting the pressure at the pump, but 5.5psi was the most it would do.  When I turned on the motor, it went back to 8.  I messed with the regulator and got it to 5.5 again, but then it jumped up to 9 for no reason.  My pump is a Mallory Comp 110 and I'm using the recommended Mallory return style regulator.  I'm using a Summit brand relay kit for the pump and I'm using the post on the starter for power.  It has worked fine until today.  The only thing that changed was that I added ground straps to the heads and block, and I removed/replaced the accessories.  I'm sure it was something that I wired wrong.  What could I have done wrong? 

[VileZambonie]

You can check the current in the pump circuit with an ameter to verify the pump is working correctly. If it is I would make sure you have no restrictions etc. If ok I'd replace the regulator.

[monte85]

i used the mallory return style regulator when i converted my truck from throttle body to carb. i had the stock in-tank pump and everytime i started the truck i would always have a different psi on the guage. i was getting jumps of like 3 to 12 psi. i would set the regulator to about 6 or 7 and when i restarted the next morning or even after going into a store. i used a guage on the feed line showed steady pressure from the pump. i read somewhere that this was a problem for these regulators.

[78ScottsdaleK20]

Just a suggestion, and it may sound elementary. Have you verified any fuel lines are not kinked or nothing is pinching the line?

I too had the fluctuating fuel pressure, and I checked the fuel line at the pump and it was kinked restricting flow. Installed a different fitting eliminating the kink, and fixed the problem.

My personal experience...

[jonkmbll89]

When any kind of trash gets into the regulator they will jump to different pressures. You can take it apart and clean them or buy another one.

[team39763]

Yes, I checked for leaks,kinks, and sharp bends.  All the lines are new.  I think it's the regulator, since it was the only secondhand part in the system.  I'm gonna pull it apart and maybe buy a rebuild kit for it.  The voltage at the pump checked out good.

[Captkaos]

Tee in before the regulator and see what the pressure from the pump is?  If it is normal for the pump before the regulator that will let you know for sure...