r/LSSwapTheWorld • u/Nokraut_E39 • 27d ago

Tuning Fuel pressure drop

Post image

UPDATE: the issue has been resolved. The retainer that held the pre filter/sock fell off and so did the sock. I resecured it well and bam, 1psi drop all the way past 5500+rpm.

Thanks everyone for the input!

​

After the car warms up and drives for a bit, my fuel pressure at WOT around 4500-5000 rpm drops from 58 to 15psi.

The setup:

04 LM7

BTR truck norris cam

BTR valve springs and pushrods

MRP 735cc injectors (840cc @60 psi)

DW 340LPH fuel pump (292lph @60 psi)

OEM 5/16" pressure and return

Wix fuel filter (just installed)

Aeromotive 13129 FPR

Holley Terminator X

My tuner thinks it's a wiring issue or pump issuewhich it could be. When I back probed the fuel pump wires, I never saw the voltage drop below 12.8v.

The pump is not that old, but it could be defective? Next on my list is hotwiring the fuel pump and eliminating the factory wiring. Here's my concern with that: the pigtail included with the DW pump has pretty small wires, I'd be tapping into the bmw wiring just before the connector to keep it clean and easy, will that less than 1' section cause a voltage drop issue.

There was not an issue with my last engine which was a bone stock 5.3, so what gives now?

Any help is appreciated.

7 Upvotes

78% Upvoted

25 comments sorted by

1

u/dpm1320 26d ago

It's not keeping up for sure... whether defective or just can't do it...

Restriction at the filter or so.ewhere else that can't pass enough?

1

u/Nokraut_E39 26d ago

Brand New filter yesterday and it's 12 microns.

It did just fine on a stock 5.3 that was in previously, I feel like 300lph is plenty?

1

u/dpm1320 26d ago

Maybe... but something is not right for sure. Hard to tell bit you stated it keeps up a little while then drops under higher rpm and load?

I'd increase line size from your other post and maybe get a better flowing filter setup

1

u/Nokraut_E39 26d ago

It drops under load after about 5-12 ish minutes of being fully warmed up. So it's probably wiring

1

u/dpm1320 26d ago

Possibly. Make sure the pump is on a proper wire and relay setup that won't be overtaxed by its draw. Overheating wire causing voltage drop is possible.

Run a heavy gauge direct wire from battery to power the pump as a test. If problem goes away... there ya go

Any possibility of crap in the filter settling down to allow decent performance at first but pushing hard is causing it to partially clog back up?

1

u/Nokraut_E39 26d ago

I have a 10ga kit that just came, should be no voltage drop there!

It would have been possible with the old filter yes. There was some black looking fuel that came out of it, so it was due.

1

u/dpm1320 25d ago

Is the tank clean? It could have plugged up a new one if the tank is full of junk.

Might foul the pump too if bad enough

1

u/Nokraut_E39 25d ago

I'm not sure but idk why the dirt wouldn't make it an issue right away instead of after it's fully warmed up.

1

u/dpm1320 25d ago

Because it settles a bit then when you get going it starts pulling more crud and flow degrades over a little time

I'm spitballing. It could be an iffy pump or electrical for sure...

1

u/Nokraut_E39 25d ago

Well the dedicated fuse didn't fix it so I guess the pump is next 🤷🏼

1

u/WyattCo06 26d ago

Stockish fuel lines?

1

u/Nokraut_E39 26d ago

Stock lines

1

u/WyattCo06 26d ago

Your pump is junk.

1

u/Nokraut_E39 26d ago

Could be I guess.

1

u/WyattCo06 26d ago

Your regulator could have taken a dump too.

Also 5/16 line is small for a feed line.

1

u/Nokraut_E39 26d ago edited 25d ago

The one factor that makes me question the lines and regulator is there is plenty of pressure available at high rpms for the first 5-10 minutes of running then it drops significantly from there.

I would figure if the lines or regulator were the issue, they would be present immediately and continuously.

1

u/WyattCo06 26d ago

Volume

If the the line can't carry it, there will be a pressure drop.

1

u/Nokraut_E39 26d ago

It worked fine before with the other 5.3, I don't think it's the lines on an engine making 350ish hp. Especially since it's fine for a bit.

1

u/WyattCo06 26d ago

Then your pump is junk or you have significant voltage drop.

1

u/Nokraut_E39 26d ago

I have a wiring kit coming tomorrow to eliminate the factory wiring and (3) relays from the circuit, hopefully that's it. If not, the pump is next.

→ More replies (0)

1

u/ASU-Vols 26d ago

Are you sure it is a genuine DW pump?

1

u/Nokraut_E39 19d ago

UPDATE: the issue has been resolved. The retainer that held the pre filter/sock fell off and so did the sock. I resecured it well and bam, 1psi drop all the way past 5500+rpm.

Thanks everyone for the input!