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EFI Fuel Pressure Drop ~5psi

General engine tech -- Drag Racing to Circle Track

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ctk30
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EFI Fuel Pressure Drop ~5psi

Post by ctk30 »

I had my car remote tuned a couple weeks ago and the tuner noticed my pressure was dropping around 5 psi under throttle and that I needed to get this fixed. I've recently changed the fuel filter and added a better 8an vent to the system and still have the same issue.

The pressure will drop free reving as well. Goes from 45 psi to 40-41, from the data log it seemed pretty steady at 40 psi under load driving.

I have a magnefuel 9925 regulator and it makes a pretty bad sounding rattling noise, I probably ought to take it apart and see what's going on.

Fuel pump is an aeromotive 340. Car should be around 500 hp and ahs holley terminator TBI. it's a 5.7 LS1 with sold roller and jones cam

Part of me thinks the fuel filter doesn't flow enough but I'm not sure, it's just a cheap summit one. This seems like the biggest one I can find with the stainless mesh https://deatschwerks.com/products/8-03- ... d248&_ss=r

It's wired from a relay from the front of the car getting power from the battery and alternator, there are a good amount of connections that I can possibly check though. I thought about wiring in something so I can measure voltage and amperage.

Any other ideas?

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Re: EFI Fuel Pressure Drop ~5psi

Post by BOOT »

I got a summit one that has a large housing(bout 6") but tiny inside filter and I'm gonna return it. Even the cheap ebay brand prefilter I got for my budget beater truck has a larger element.

Also if it does it free revving, have someone test the voltage at the pump while
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Re: EFI Fuel Pressure Drop ~5psi

Post by dannobee »

A couple of things.

Check for voltage drop at the pump as BOOT suggested. Also plug the bypass/return line to check max pump output. It should easily exceed the regulated pressure.
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Re: EFI Fuel Pressure Drop ~5psi

Post by Rick! »

340 - in tank pump? Sock installed correctly?
Mounted so it has a sump to pull from?
Where is the return line pointed at in the tank? You don't want it pointed at the pump pickup, in most cases.
Driving it with a relay? SS relay?
Power is coming from a rear or front mounted battery?
Measuring voltage drop from the battery positive terminal to the positive terminal on the pump will tell you if your feed is proper. By rights it should be within 0.3-0.4v of battery voltage when you snap the throttle.

Aeromotive has several options for a 10 micron fuel filter and housing.
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Re: EFI Fuel Pressure Drop ~5psi

Post by Belgian1979 »

Mine has a similar behavior. Never really found the cause. I bet it has to do with the injectors opening and closing. Set the smoothing factor higher on the pump pressure regulator if it is used for any kind of calculation.
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Re: EFI Fuel Pressure Drop ~5psi

Post by allencr267 »

Where is the pressure read & regulated?
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Re: EFI Fuel Pressure Drop ~5psi

Post by ctk30 »

yes, I think voltage drop is my next test. I can probably just take the element out of the filter and run it for a little to test the filter.
Rick! wrote: Mon May 03, 2021 11:07 am 340 - in tank pump? Sock installed correctly?
Mounted so it has a sump to pull from?
Where is the return line pointed at in the tank? You don't want it pointed at the pump pickup, in most cases.
Driving it with a relay? SS relay?
Power is coming from a rear or front mounted battery?
Measuring voltage drop from the battery positive terminal to the positive terminal on the pump will tell you if your feed is proper. By rights it should be within 0.3-0.4v of battery voltage when you snap the throttle.

Aeromotive has several options for a 10 micron fuel filter and housing.
340 is in tank, the sock is hopefully installed correctly as far as i know. It is mounted in a box with check valves

the return line goes straight down with a piece of hard pipe to about 2" above the bottom of the fuel cell, not into the box

It has a relay, not a solid state. I did have an issue inutility where the relay was bad and I had to change it to get the fuel pump to work.

Battery is behind seat. Power for the fuel pump comes about 8 ft of cable away at a cut off switch.

I'll get a couple deutsch connectors and wire in a way to check voltage drop

Those aeromotive filters seem decent as well
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Re: EFI Fuel Pressure Drop ~5psi

Post by ctk30 »

allencr267 wrote: Mon May 03, 2021 5:11 pm Where is the pressure read & regulated?
Read at the TBI, regulated about 14ft away at the fuel tank
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Re: EFI Fuel Pressure Drop ~5psi

Post by ctk30 »

I did talk to Magnefuel, they said that the noise my regulator is making is either low flow or air in the fuel and could damage the regulator

If its low flow that would make sense with the pressure drop under throttle

I'm really trying to avoid taking the fill plate off at this point still, seems like a mess
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Re: EFI Fuel Pressure Drop ~5psi

Post by RednGold86Z »

ctk30 wrote: Mon May 03, 2021 6:50 pm
allencr267 wrote: Mon May 03, 2021 5:11 pm Where is the pressure read & regulated?
Read at the TBI, regulated about 14ft away at the fuel tank
This is your problem. Not a big problem if it's fully closed loop AFR control, and if the injectors are a bit larger than necessary. If you were to read it at the regulator, it would likely be constant ish (but that's not important). If you were to put the regulator up near the engine, it would be much less drop, especially if you filter the inlet to the regulator and not the outlet.
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Re: EFI Fuel Pressure Drop ~5psi

Post by 2seater »

In many OEM installations the pressure regulation is actually downstream of the injectors
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Re: EFI Fuel Pressure Drop ~5psi

Post by ctk30 »

I wired in a voltage meter before the pump, not sure if that was the correct method. I'll try to go from the battery on the terminals as well to see voltage drop. My ECU was reading 12.2 with the car off as well

I thought I read on the holley forum about turning the pressure up to 60 psi if the regulator is further away. I'll have to search more

How I wired it in

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With the pump running

Image

With the pump unplugged

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Re: EFI Fuel Pressure Drop ~5psi

Post by Racer71 »

Regulator needs to be immediately after rails and vacuum sourced to bump pressure. It should rise approx 5lbs when vacuum goes away ie: when you step on gas.
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Re: EFI Fuel Pressure Drop ~5psi

Post by NewbVetteGuy »

2seater wrote: Tue May 04, 2021 12:45 pm In many OEM installations the pressure regulation is actually downstream of the injectors
Sure but within a few inches to a couple of feet of the fuel rails, not like 14 feet, right? (Real question).


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Re: EFI Fuel Pressure Drop ~5psi

Post by allencr267 »

14ft of what?
Bends, restrictions, 90's, fittings?
In the below pic, is that the line, 3 or 4/16ths ID lean looking stainless.?
ctk30 wrote: Tue May 04, 2021 8:26 pm Image
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