Fuel pump broke or undersized? - Vintage Mustang Forums
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post #1 of 11 (permalink) Old 09-06-2017, 03:36 PM Thread Starter
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Fuel pump broke or undersized?

I've been running this Holley Mighty Mite fuel pump for three years:
https://www.holley.com/products/fuel...s/parts/12-427

This year it started acting fuel starved so I swapped out all the filters and fixed that problem. However, I noticed that even after the filter changes I was having fuel pressure drops under WOT. It idles at around 4.5 psi, but after an eighth mile of WOT it will drop down to under one psi (at which point I let out of the throttle).

I never checked it when new, so now I don't know if the pump is petering out or if it's undersized. I would like to just get another one (for ease of installation), but I don't want to replace undersized with undersized and get the same problem.

Engine is a mild port 94 5.0L with a crappy 289 edelbrock performer intake and mid length headers. Well under the 400hp that the pump is advertised for.

So, replace or upgrade?

1966 Mustang 302ci 4r70w, csrp front discs, 8.8 rear, EPAS, Fitech for fuel
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post #2 of 11 (permalink) Old 09-06-2017, 10:18 PM
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What type of fuel pressure gauge are you using and where is it located at? What carb are you using?

Last edited by cmefly; 09-06-2017 at 10:24 PM.
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post #3 of 11 (permalink) Old 09-07-2017, 01:45 PM Thread Starter
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What type of fuel pressure gauge are you using and where is it located at? What carb are you using?
I just have a generic parts store gauge and I put it between the filter and carb. I'm using a summit brand carb. The carb is like an autolite 4100 I think. 600cfm with vac secondaries. Rebuilt three years back.

I'm pretty sure the gauge works cause it was showing low psi and rapid drop-off of psi consistent with how it was running before the filter changes. After the filter changes it did show improvement and slower psi drop.
My gauge also shows vacuum. I'm getting about -17 at idle

1966 Mustang 302ci 4r70w, csrp front discs, 8.8 rear, EPAS, Fitech for fuel
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post #4 of 11 (permalink) Old 09-10-2017, 02:31 PM
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If you are running at WOT for any length of time you should seriously consider the results of an over lean condition. Is the motor worth the extra money to keep it well fed with fuel. Think about using a more capable fuel pump. It will only feed what the motor uses, much like an alternator feeding the electrical system.
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post #5 of 11 (permalink) Old 09-10-2017, 05:17 PM Thread Starter
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Originally Posted by trkpny2 View Post
If you are running at WOT for any length of time you should seriously consider the results of an over lean condition. Is the motor worth the extra money to keep it well fed with fuel. Think about using a more capable fuel pump. It will only feed what the motor uses, much like an alternator feeding the electrical system.
I'm with you totally. Are the specs on that pump I'm using adequate? Or should I look into upgrading?

1966 Mustang 302ci 4r70w, csrp front discs, 8.8 rear, EPAS, Fitech for fuel
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post #6 of 11 (permalink) Old 09-10-2017, 06:43 PM
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You may have a clogged line at the tank or a pinched fuel line. It will pass enough fuel for idle but not for WOT

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post #7 of 11 (permalink) Old 09-10-2017, 07:37 PM
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Quote:
Originally Posted by copracr View Post
I just have a generic parts store gauge and I put it between the filter and carb….."

FYI: all liquid filled gauges will show a dramatic drop off of pressure when they get HOT. The actual fuel pressure is essentially unchanged .

In other words, whatever information you want to get from an underhood gauge, get it before the motor heats up and by extension the gauge gets hot too. Once the gauge gets hot, you might as well not even look at it.


Z
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post #8 of 11 (permalink) Old 09-10-2017, 08:19 PM
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copracr, I would upgrade the pump. 32gph is simply not enough fuel for what you are doing. I just upgraded my billet Holley mechanical pump which was apparently rated at 170gph to a Clay Smith pump that puts out somewhere in the 200gph range to solve my fuel starvation problem at sustained 5,500rpm on the road course. There are plenty of inexpensive electrical pumps out there, so spend as much as you can on a decent pump. Other factors also come into play, such as fuel line size ( min.3/8"), filter capabilities to match the pump output, and whether or not to run a return style regulator/system. I am running -8 (1/2") supply and return and an Aeromotive 13301 bypass regulator. Good luck.
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post #9 of 11 (permalink) Old 09-10-2017, 10:32 PM
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What size are your fuel lines? Is the pump within 12 inches of the tank like mentioned in the specs?

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post #10 of 11 (permalink) Old 09-11-2017, 06:56 PM
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Quote:
Originally Posted by zray View Post
FYI: all liquid filled gauges will show a dramatic drop off of pressure when they get HOT. The actual fuel pressure is essentially unchanged .

In other words, whatever information you want to get from an underhood gauge, get it before the motor heats up and by extension the gauge gets hot too. Once the gauge gets hot, you might as well not even look at it.


Z
What he said
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post #11 of 11 (permalink) Old 09-21-2017, 11:31 AM Thread Starter
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The fuel tank and sending unit are new.

5/16" lines.

Pump is mounted at about mid fuel tank height and about 9" of hose from tank to pump.

I ran my gauge out from under the hood and put it under my wiper blade so it wouldn't get hot and so I could see it while driving.

trkpny2- I was wondering about the Holley claim that 32gph is good for 400hp. That seemed a little low to me, but I went with it. Now I'm wondering if it's just not sufficient for my mild 302 making maybe 275hp, lol

1966 Mustang 302ci 4r70w, csrp front discs, 8.8 rear, EPAS, Fitech for fuel
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