Unstable Fuel Pressure

[ykf7b0]

Hello,
My fuel system is giving me problems with instability and I'm turning to you guys for help. I have a Mallory 140 Series electric pump with a Mallory 4309 regulator that has a return port that I have utilized. The fuel pressure gauge is a Autometer Pro Comp and the isolator is Mallory. The fuel lines are 3/8 aluminum for feed and return. I have removed the spring inside the fuel pump as recommended by instructions. The engine is a healthy 440 with an 850 Holley in a gasoline street car. The problem is that I can set the fuel pressure at 6 1/2 psi and after a short while it might register as high as 12 psi and sometimes it will register as low as 4 psi after a drive. I thinking of blocking return line and letting it dead head to see if it helps but I want to get some sound advice before I do anything. Thanks in advance!!!

 

[lewtot184]

i would think a 3/8" return would be way too large.

 

[ykf7b0]

? anyone else?

 

[Hemirunner]

What does the spring do and why did the instructions say to remove it?

 

[67 coronet]

how new is the regulator?i have had a few problems with mallory regulators bad off the shelf.(i used to sell them)you could try blocking the return,but why would the reg let the presure rise so high?

 

[ykf7b0]

What does the spring do and why did the instructions say to remove it?

The Mallory instructions say to remove the aforementioned spring from the fuel pump if using a return style regulator. Not sure as to why but it's a Mallory pump and Mallory regulator.

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how new is the regulator?i have had a few problems with mallory regulators bad off the shelf.(i used to sell them)you could try blocking the return,but why would the reg let the presure rise so high?

Regulator was new last Summer. You have a good point regarding the high pressure reading.

 

[ykf7b0]

i would think a 3/8" return would be way too large.

Mallory instructions say use same size return as feed line.

 

[lewtot184]

Mallory instructions say use same size return as feed line.

so, i'm thinking that removing the relief spring lets the pump go full tilt boogie all the time. if the engine doesn't flood out from too much fuel at 12psi then i'd suspect the gauge.

 

[ykf7b0]

so, i'm thinking that removing the relief spring lets the pump go full tilt boogie all the time. if the engine doesn't flood out from too much fuel at 12psi then i'd suspect the gauge.

I have two gauges on it. One is inline under the hood (Redline brand) and other is inside passenger compartment. (Autometer Pro Comp) I'm thinking both gauges wouldn't be wrong. I know engine is starving at full throttle because I've ran it wide open and it will stall out after a relative short distance. Thanks!

 

[lewtot184]

I have two gauges on it. One is inline under the hood (Redline brand) and other is inside passenger compartment. (Autometer Pro Comp) I'm thinking both gauges wouldn't be wrong. I know engine is starving at full throttle because I've ran it wide open and it will stall out after a relative short distance. Thanks!

if its starving for fuel than the return is too big. pressure fluctuation could be the regulator, or possibly some fluctuations in the voltage to the pump. why not reinstall the spring stuff, block the return, reset pressure and then give it a test drive.

 

[Hemirunner]

if its starving for fuel than the return is too big. pressure fluctuation could be the regulator, or possibly some fluctuations in the voltage to the pump. why not reinstall the spring stuff, block the return, reset pressure and then give it a test drive.

If the regulator is dong its job, then having too big of a return is a non-issue. (think about it....) The regulator should be the only restriction in the line and it needs to be a restriction to actually allow pressure to build to the carb. Any lines after the regulator on the return side could be a 4" in diameter (silly yes, but hopefully you get my meaning) for all the system cares. The line from the pump through the regulator up to the carb is what needs to be sized according to demand and 3/8 may not be moving enough fuel for that application. The other possible issue could be pump related, but either way, running a return style setup takes a bunch of additional volume over a dead head setup. This is especially true with a high pressure pump as you have to bypass even more volume to relieve the pressure.

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One other thing to consider.... is the tank vented properly? I've seen tanks with no vents cause huge pressure issues with fuel systems.

 

[Jasoncoronet]

Make sure all your fuel lines are away from the heat. As fuel heats it vaporizes which can cause unstable fuel preasure or worse a vapor lock situatio. I had to add a phenolic spacer to the carb to help keep heat down. This helped my problem. Also a faulty regulator or guage will cause this issue. I also think it is pretty normal to fluctuate a little. Just set it cold and forget it.

 

[Meep-Meep]

I have no experience with the equipment you are using but pressure is pressure so I'll offer this. The pressure will vary as the float drops so perhaps setting the pressure reg with the pump dead headed will establish a maximum. From there you should only see reduced pressure in the line based on demand.

 

[lewtot184]

If the regulator is dong its job, then having too big of a return is a non-issue. (think about it....) The regulator should be the only restriction in the line and it needs to be a restriction to actually allow pressure to build to the carb. Any lines after the regulator on the return side could be a 4" in diameter (silly yes, but hopefully you get my meaning) for all the system cares. The line from the pump through the regulator up to the carb is what needs to be sized according to demand and 3/8 may not be moving enough fuel for that application. The other possible issue could be pump related, but either way, running a return style setup takes a bunch of additional volume over a dead head setup. This is especially true with a high pressure pump as you have to bypass even more volume to relieve the pressure.

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One other thing to consider.... is the tank vented properly? I've seen tanks with no vents cause huge pressure issues with fuel systems.

sorry, but i don't get it. i don't think there's any way on the planet that a 140 mallory can support a 3/8" supply and return. i've seen them on 400-500 gallon pumps.

 

[kidmopar]

mallory regulators are known to go bad, try the holley regulator in its place , also ive hever seen a pump needing the spring taken out. if the pump supplies 12 psi without a reg then thats why a reg is needed, to keep it down, the spring in the pump keeps the pressure constant, the only diiference in a holley blue and holley red pump is the spring, one uses the reg while the other does not, im sure if u put the spring back in it will act right if the reg isnt bad.

 

[ykf7b0]

sorry, but i don't get it. i don't think there's any way on the planet that a 140 mallory can support a 3/8" supply and return. i've seen them on 400-500 gallon pumps.

? I'm not sure I understand what you meant to say here. Their instructions advise 1/2 supply and return on some applications where long fuel line are utilized.

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mallory regulators are known to go bad, try the holley regulator in its place , also ive hever seen a pump needing the spring taken out. if the pump supplies 12 psi without a reg then thats why a reg is needed, to keep it down, the spring in the pump keeps the pressure constant, the only diiference in a holley blue and holley red pump is the spring, one uses the reg while the other does not, im sure if u put the spring back in it will act right if the reg isnt bad.

Hey kid!!! It's great to hear from you!!! Yea these Mallory 140 Series are unique. They have a bypass plug and spring inside on the bottom portion of the pump and the instructions are clear about removing the spring when using a return style regulator with return fuel line. I only chose the Mallory regulator because it's the only one I could find at a decent price with 3 ports. (5 total counting inlet and return) I need it for my nitrous fuel delivery because I don't have a stand alone fuel system for the nitrous. I like the Mallory pump because it is much quieter in my street roadrunner than the Holley Blue or the Carter High Pressure/High Flow. I'm thinking at this point my regulator is not up for the job.

 

[ykf7b0]

Are there any recommendations for a quality 3 port regulator?

 

[493charger]

The 4150 Mallory is internally bypassed, so deadheading it is ok. I would plug the return and set the pressure to about 5-6 and see if this solves your problems. If so, you have a regulator problem. If not, you have a fuel delivery problem.

Also, make sure your fuel pump inlet is at least 4-6 inches below the tank/cell. Most pumps are designed to push fuel, not pull. If the pump is starving for fuel, so will the engine.

I also run a large 80-100 micron inline pre-filter before the pump, with a 10 micron canister filter after it. This keeps junk from the tank from getting in the pump. If a piece of debris gets stuck in the internal bypass, fuel pressure will be off.

 

[ykf7b0]

I have inspected the diaphram in the regulator and it looked great. I blew the return line out to clear any possible obstructions. I can set the regulator but the pressure falls way down upon consistant high rpm's, (4000 or higher). I have recently ordered a new fuel filter element for my 35 micron Earls's inline filter (#230103) Could it be this filter is too restrictive even if clean and clear??

 

[ykf7b0]

UPDATE; Fuel pressure is much more stable with the small Earl's 35 micron inline filter removed. This little filter is apparently much to restrictive for my application. I'm going to install a Fram large cannister type filter if I can find the room underneath above the axle housing. I want to thank everyone who chimed in and gave their suggestions and opinions of what was causing my fuel pressure to be so unstable!!!