Fuel Pressure Gauge Needle Fluctuating ?

[Mike Gaines]

I need help with this.
I have a Mallory 140 Electric Fuel Pump with a Mallory Return Style Fuel Pressure Regulator and a Aeromotive Liquid Filled Fuel Pressure Gauge.
Problem:
I can set the fuel pressure at exactly 7.5lbs (as per Holley's Instructions) with the fuel pump turned on but the motor NOT running. It stays steady at 7.5lbs with the pump merely running off the battery (and the motor not yet started).
When I start the motor the Fuel Pressure Gauge fluctuates (bounces) between 6lbs and 9lbs, sometimes not a lot, but at other time it bounces quite a bit.
I mounted a second Liquid Filled 1.5" Fuel Pressure gauge on the "cowl" so I could see it during a 1/4 mile run. There is also a 1.5" liquid fuel pressure gauge mounted next the regulator on the regulator mount beside the carb. Both gauges bounce.
I aimed my "Go-Pro" camera at the cowl mounted liquid filled gauge as I made a 1/4 mile pass. The car seemed to run strong and I didnt feel any "surging" or "motor stalling out, or anything like that but all the way down the strip the Fuel Pressure gauge fluctuated between 6-9lbs pretty violently.
I saw a Painless Performance Fuel Pump Relay Kit in the Summit Catalog (#50102 for about $35) that says it will "prevent voltage drops at the pump thus maintaining constant fuel pressure".
Question: Have any of you guys had any similiar experiences and/or used this relay kit.
I am not sure if I merely have a loose connection at my fuel pump or somewhere else in the fuel pump wiring or if the fuel pump is defective...or what ??
Help, Please

 

[Budnicks]

IMO You should always use a relay on any added automotive electronics...

It keeps up a constant voltage to the system,
there's always 12vts {or what ever voltage} constant,
with less fluctuations in all electronics, lights, pumps, fans, radios, ignition even
including the gauges & especially the fuel pump, that can draw allot of amperage...

BUT that doesn't mean, that's your issue either...
It wouldn't hurt thou

not the only way either, here's a few examples,
even a crude example of one I did for my cars aux. switches, pumps & electronics
good luck

 

[Cranky]

Do you have another regulator you can try?

 

[Mike Gaines]

Do you have another regulator you can try?

Cranky....I had a Mallory Fuel Pressure Regulator on the car when the issue started. I bought a BRAND NEW Mallory (now MSD) Regulator and it didnt change anything. I also bought a new Aeromotive Fuel Pressure Gauge (my old one wasn't Liquid Filled) and the same problem of fluctuation....so ruled out the Regulator and the Gauge....Leaves the wiring to the pump and the pump itselt as the culprit, I guess.

Budnicks: I understand the Fuel Pump Relay Kit would be a good option to install and I probably will do that...but as you say, I dont know if that will remedy my fluctuation problem.

I would think it has to be the
1) wiring to the pump
2) the pump intself or
3) maybe the fuel lines from the pump to the fuel pressure regulator.

I was hoping someone had a similiar situation and could point me in the right direction before I tear into all three items. Thanks

 

[ozymaxwedge]

Mike is your fuel pressure regulated by the amount of return ?

 

[lewtot184]

i bet your getting a pulse from the return.

 

[ozymaxwedge]

Have you removed the bypass plunger and spring in the pump ?

 

[ozymaxwedge]

http://mallory-ignition.com/media/i...ssure_regulator_3_point_return_style_4309.pdf

 

[Shorthorse]

I tend to believe the issue is with the return system.
I am not an electrical engineer but I spent my whole working life with low voltage DC current. While I understand the use of relays in street and more modern cars, I have an issue with using relays especially in race cars. First of all, the 12V supply is from the same single source. The general delivery of that 12V's is through a set of contacts. Whether it is through a relay contact or the contacts of a switch the same considerations apply. That is, the supply must be constant and within a proper voltage range. The contacts are more than adequate to carry the load and that the wire used to carry that load is also more than adequate. The path is still the same regardless. The power is delivered through a set of contacts. I fail to see how the contacts of a relay are better or have some magic ability to correct any issues with the power source that the contacts of a relay doesn't have. The fact is, contacts only pass the power source to the device.
My issue is with the design when using relays and that is the introduction of additional points of failure. I once tried to figure out the difference in those points of failure when using relays vs a single toggle switch. It's rough but this what I came up with. The red dots and arrows are an attempt to show the most common possibilities. Again, my thoughts are mostly confined to race cars and the need for reliability. Part of having a race car is the eternal search for the weakest link.

 

[hangn0ut]

The fact the pressures rises to 9psi rules out a bad connection or intermitent voltage supply. Race cars shake a lot what with solid motor mounts,rear suspensions with no rubber bushings,fuel pumps bolted down etc. Even the gauges are solid mounted i suppose. Mine are and they do the same,not as much though. The gauges may be liquid filled but the needles are very thin in the direction of travel. Try a lower pressure and see if it starts to lean out. Lower a bit at a time until it shows its going lean. Max pressure tends to aerate the fuel in the bowls so less might be better for you. Only one way to find out.

 

[Mike Gaines]

Mike is your fuel pressure regulated by the amount of return ?

Ozy...I don't understand your question.
I merely set the fuel pressure in the regulator and that is it. Nothing in the instructions (or literature) talks about "the amount of the return". I presume that the fuel the carburetor is NOT using merely gets return to the tank via the bypass return line.

- - - Updated - - -

http://mallory-ignition.com/media/i...ssure_regulator_3_point_return_style_4309.pdf

Ozy...YES, I have removed the bypass spring from the fuel pump.

- - - Updated - - -

i bet your getting a pulse from the return.

lew...that sounds logical...a "pulse" would cause the fluctuation...but how does a pulse originate ?

- - - Updated - - -

The fact the pressures rises to 9psi rules out a bad connection or intermitent voltage supply. Race cars shake a lot what with solid motor mounts,rear suspensions with no rubber bushings,fuel pumps bolted down etc. Even the gauges are solid mounted i suppose. Mine are and they do the same,not as much though. The gauges may be liquid filled but the needles are very thin in the direction of travel. Try a lower pressure and see if it starts to lean out. Lower a bit at a time until it shows its going lean. Max pressure tends to aerate the fuel in the bowls so less might be better for you. Only one way to find out.

hangnout...I guess what you are saying makes sense. I have set the pressure to 7.5lbs and then the pressure goes "up" to 9lbs when it fluctuates then it couldn't be a "drop" in voltage to the pump causing the fuel pressure to "rise".
It does "drop" to about 6lbs when it fluctuate but then it goes "up" to 9lbs as part of the fluctuation pattern.

Maybe it could be merely the motor/car/etc normal "shaking" causing it....but then, I would think EVERYONE would have the same problem ???

Help, Help, Help

 

[GTX MATT]

What are you running for a pickup? If stock I would check the sock on and make sure you don't have any hard kinks/bends between the pump and tank. I had a similar issue with a mechanical pump and it turned out to be a restriction I had before the pump.

https://www.youtube.com/watch?v=uNPGGy8jBks

 

[66Satellite47]

Mike, could it be related just to the needle/seat actuation? That dynamic seems to be quite variable.

 

[bm02tj]

It is likely the droop and recover of the pressure control trying to maintain the set pressure with the needle and seat taking fuel then closing
Gas is a liquid and is solid if you want your gauge to settle down add a restricted orifice to it to slow or dampen the gauge more

 

[cdr]

what size return line do you have ?

 

[Mike Gaines]

what size return line do you have ?

CDR ... I have 6an (3/8) line from the pump to the regulator and the sane size for the bypass line...I have already talked to Mallory Tech and they say that is good to go.

- - - Updated - - -

Mike, could it be related just to the needle/seat actuation? That dynamic seems to be quite variable.

66Sat....How would I know if it needle and seat ?

- - - Updated - - -

It is likely the droop and recover of the pressure control trying to maintain the set pressure with the needle and seat taking fuel then closing
Gas is a liquid and is solid if you want your gauge to settle down add a restricted orifice to it to slow or dampen the gauge more

?????

 

[Longroof]

i would put a bigger return line on it. Always ran bigger return lines than feed lines on all my cars and my uncle and cousin's super stockers... it just makes the fuel system work better.

 

[ozymaxwedge]

CDR ... I have 6an (3/8) line from the pump to the regulator and the sane size for the bypass line...I have already talked to Mallory Tech and they say that is good to go.

Maybe Im going to learn something here but having -6 feeding 2x -6 carb lines and a -6 return seems all wrong to me.
I run -10 feed -8 bypass and -6 lines to carb.

Again I might be going to learn more but I am shocked with the pump size as well. Magna fuel rate there 275 GPH pump for "up too" 750 hp . How can a 140 with -6 feed be right ? Ive been around the drags for a while (not as long as you Mike) and if I was doing the fuel system for your car Mike it would go roughly like this : -10 tank to pump, 200+ gph pump, -8 feed to hi flow reg, -8 return and -6 to carb.
Just my thoughts.

 

[cdr]

for carb systems if you run a # 6 supply it need an # 8 for the return, that means all fittings,no adapters.

- - - Updated - - -

efi is different because it is high pressure.

- - - Updated - - -

also the liquid filled gauges , when they get hot do not read correct.you have to open the little rubber cap on the side of the gauge to equalize the pressure that has developed from the oil expanding.

 

[383man]

Just curious as to if you have tried a non liquid filled gauge ? I here the liquid filled gauges will bounce around some. So it makes me curious if a non liquid filled gauge will read the same ? Ron