Fuel Pressure Not Keeping

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BrokenPony

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Aug 12, 2010

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Aug 18, 2010

#1

• Aug 18, 2010

• #1

'93 GT - The fuel system doesn't maintain presure. Key on, pump pressures up the system but as soon as the pump stop, the pressure is gone. Within 3 seconds of the pump turning on, there is no pressure in the system. What should I be looking at?

 

H

Haus

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Jul 20, 2005

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Aug 18, 2010

#2

• Aug 18, 2010

• #2

Im dealing with the same problem. Even with a brand new walbro pump and new FPR.

 

L

Lawbreaker5.0

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Aug 18, 2010

#3

• Aug 18, 2010

• #3

I'm in the same boat as y'all. Brand new 255lph and new holley adj FPR. I'm thinking mine might be related to my FPR since it didn't hold vacuum but who knows. Summit is sending me the return box and I'll ship mine off asap so I get the next "new" one. Subscribe to my thread underneath this one and I'll post my results there.

 

Mustang5L5

That is…until I whipped out my Bissell

Mod Dude

Feb 18, 2001

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Aug 18, 2010

#4

• Aug 18, 2010

• #4

Does the system maintain pressure when running?

It's actually pretty common and I believe the fuel pump is the culprit. Not quite convinced it's an actual problem though. Mine bleeds off slowly after stopping the pump. 5 mins and it's at 0

 

B

BrokenPony

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#5

• Aug 18, 2010

• #5

It holds while running. But drains instantly. Supposed to hold for 5 minutes or so.

 

L

Lawbreaker5.0

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Aug 18, 2010

#6

• Aug 18, 2010

• #6

My convertible holds for about 20-30 min with holley FPR and 190lph and the coupe drops within secs so I know something isn't right

 

HISSIN50

"How long does it take to get help in here?

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#7

• Aug 18, 2010

• #7

Aftermarket FPRs generally drop pressure quicker than OEM FPRs.

As said, as long as pressure is adequate while running, it's no biggie. You don't want pressurized fuel in the lines when the car isn't running anyhow.

 

jrichker

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Aug 19, 2010

#8

• Aug 19, 2010

• #8

As long as the pump maintains pressure while it is running and there are no leaks, the drop in fuel pressure when the pump turns off is not a problem.

 

B

BrokenPony

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#9

• Aug 19, 2010

• #9

Fuel pressure is fine when it's running. Was just wondering if the quick drop was an indication of an issue someplace. Thanks guys.

 

jrichker

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#10

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• #10

After some more though, now I understand why the line is supposed to retain pressure after shutdown. It prevents vapor lock and hard starting on a hot engine.

Here’s why: when you pressurize a liquid, you raise the boiling point. It doesn’t matter what the liquid is water, oil, gasoline, brake fluid. That’s why your car has a pressurized radiator: instead of water boiling at 212 F, with 14-15 PSI pressure, it boils at 250 F. Gasoline pressurized to 40 PSI would react similarly, and its boiling point would go up beyond the 180F-220F commonly found in engine compartments.

A fuel supply line full of nothing but vapor would make the car hard to start until the vapor bled off during cranking, or the pressure inside the fuel supply line forced the vapor to condense back to a liquid.

 

L

Lawbreaker5.0

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#11

• Aug 20, 2010

• #11

Alright! jrichker now give us a write up on the process of diagnosing the fuel system

 

jrichker

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#12

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Lawbreaker5.0 said:

Alright! jrichker now give us a write up on the process of diagnosing the fuel system

Click to expand...

I already have four tech notes on troubleshooting the electrical part of the fuel system . One for 86, another for 87-90, another for 91-93 and one for the 94-95 Mustangs. I guess the next project will be to do a tech note on the fuel plumbing/pump and regulator troubleshooting.

I have an occasional hiccup with my current 5.0 that seems to be common to the previous 89 GT I had. I have heard that others have had the same experience. The fuel system mysteriously quits working and I am stranded until it starts working again. The fix seems to be to bang on the fuel pump relay. Troubleshooting in a hot parking lot when you in a hurry has given me a new appreciation of the difficulty of diagnosing the circuit. I am seriously thinking of building a plug in relay circuit tester using an old relay housing, a relay socket and 4 LEDS to show the state of the voltages on the relay pins.

 

mob

the guy who hits on his mom

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#13

• Aug 20, 2010

• #13

jrichker said:

I am seriously thinking of building a plug in relay circuit tester using an old relay housing, a relay socket and 4 LEDS to show the state of the voltages on the relay pins.

Click to expand...

That would be super cool, write up and pictures please

 

L

Lawbreaker5.0

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Sep 3, 2010

#14

• Sep 3, 2010

• #14

i replaced the adj one with my stocker and everything is normal. PSI holds steady when i prime the car and hasn't dropped under 30 psi and its been close to an hour.

 

B

brian69

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Jun 29, 2010

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Sep 5, 2010

#15

• Sep 5, 2010

• #15

On factory fuel systems it is always the pump.