Fluctuations in fuel pressure sending me mad

[MR4V]

Hi All
Need some help with fluctuating fuel pressure.
This is a all new dead head system and is in a Duster with new sender with 1/2 inch pick up, 1/2 to Holley 12-125gph pump, 1/2 to Holley 12-845 Billet reg and 1/2 to E85 Quick Fuel Q850.

Have great pressure from idle to about 3500rpm (6psi) and then it drops as low as zero and then back to 6psi.
After a run on the dyno it fluctuates at idle up to 5psi. Before and during the run float bowls are at the correct level.

I've rechecked the hardware and flow and pressure to the reg is great. Yes volts to pump are spot on.

This is starting to get to me as I can't find the fault

 

[KnuckleDuster]

How close is your feed line to the headers?

Mine was vapor locking at WOT on the street and dropping to zero psi. I had to shield the feed and run a return to fix it.
If that's the problem, it would be even worse on a staionary car on a dyno. Drove me nuts chasing it.

 

[brian6pac]

I had a problem of sucking air on the suction side getting air in the regulator, pressure kept dropping off. Used better clamps on the suction side fixed it.

 

[67Dart273]

I would take a REALLY careful look at the suction side

1.....As above sucking air, IE mis--located pickup or plugged / "sucking shut" filter sock

2.....What is inlet, hose? tubing? Hose sucking shut or INTERIOR of hose, lining sucking in

3....Certainly inspect pump if it can be taken apart, for dirt, junk, whatever.

4....Location of pump where is it? Height? Above/ below tank and length of suction run

By "dead end" you are saying? there is no return line? Here's what this means.........The pump agitates the fuel at low throttle settings and creates it's own vapor lock so to speak. If you do have a (mentioned above) heat problem, this allows the fuel to "sit there" by the heat source rather than flow on by, so you could be creating pockets of vapor.

 

[MR4V]

Thanks all.
Yes this system has no return line and is vented via the fuel cap. I've removed the cap and still have the same issue.

I to believe it's the suction side, the flexible line is new, E85 compatible and as too all the other lines.
I have checked to see if any line has delaminated and all lines check out.
All lines are -8 with 3/8 inlet and outlet on the pump as per manufacturer.
No sock on pick up and the pump is located 10" away from tank and slightly lower.

Haven't inspected the pump so that's now the next job.

If the line between the pickup and the pump sucks air wouldn't also leak fuel?

 

[KnuckleDuster]

Are the lines braided steel covered rubber, hardline, or ???

You have headers?

Your lines are how far from the collectors?

5/16", 3/8", 1/2", or dash anything won't help if you are having a weenie roast.

If you have new lines and confidence that you connected them right, I would be looking at heat as a culprit.

I don't know crap about E85, but the ethenol tainted pump gas here vaporizes if you stare at it too hard.

Ate you having symptoms like this?

 

[MR4V]

Thanks for everyone's help.
Problem fixed, it turned out to be a faulty fitting that was not sealing on the suction side, so under WOT it was sucking air.
Now have a constant 6psi.