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  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • 1994 - 1995 Specific Tech

Slow Fuel Pressure

  • Thread starter Thread starter Altered
  • Start date Start date Jun 22, 2022

Altered

New Member
Apr 24, 2021
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Iowa City
Jun 22, 2022
#1
  • Jun 22, 2022
  • #1
I have a stock '95 Mustang GT. Recently it started to feel down on power. While doing a fuel pressure test, I noticed that the needle on the guage was moving very slowly and eventually stopped at 32 to 34 psi at idle with no change when the motor is revved. It seems as though the car is struggling to build fuel pressure. Any ideas what could cause this? I couldn't find any fuel leaks while doing the pressure test. The system also held pressure when the car was off. As of May 2021 the car got a brand new 190lph Walbro pump, and brand new Wix filter. When changing the pump I inspected inside the tank for debris and it was clean. The car has also never been ran much bellow 1/4 tank since the pump was replaced. Any help is appreciated.
 

96pushrod

I think they're a little easier to get off
5 Year Member
May 15, 2018
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Savannah
Jun 22, 2022
#2
  • Jun 22, 2022
  • #2
Assuming the vacuum line is hooked up to the regulator, that’s pretty normal pressure. If the engine isn’t really under load then the pressure won’t move much when you rev it. Only way to check is to monitor pressure when you’re driving and you floor the gas. It should jump to around 39ish.

I’d put a timing light on it with the spout disconnected and see where it’s at as well.
 

Altered

New Member
Apr 24, 2021
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Iowa City
Jun 22, 2022
#3
  • Jun 22, 2022
  • #3
You're right, the end pressure seems okay if not maybe a touch low. I'm currently more perplexed by the fact that the engine had to run for around three to five minutes before the guage read above 30 psi
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
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polk county florida
Jun 22, 2022
#4
  • Jun 22, 2022
  • #4
Check the fuel filter, the tanks at fuel stations can get stirred up and transfer the junk into your tank..
 

Altered

New Member
Apr 24, 2021
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Iowa City
Jun 22, 2022
#5
  • Jun 22, 2022
  • #5
That's next on my troubleshooting list. I'll be a bit concerned though if the filter got clogged up that quickly.
 

manicmechanic007

5 Year Member
Sep 26, 2017
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Roy, Utah
Jun 23, 2022
#6
  • Jun 23, 2022
  • #6
Double check that slow to build fuel pressure with a different gauge
If the same replace the fuel pump again
 

manicmechanic007

5 Year Member
Sep 26, 2017
2,526
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Roy, Utah
Jun 23, 2022
#7
  • Jun 23, 2022
  • #7
Replace the module assembly if it needs a fuel pump
You may have a bad one way valve or fuel damper
 

Altered

New Member
Apr 24, 2021
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Iowa City
Jun 23, 2022
#8
  • Jun 23, 2022
  • #8
manicmechanic007 said:
Replace the module assembly if it needs a fuel pump
You may have a bad one way valve or fuel damper
Click to expand...
The entire fuel pump assembly was replaced at the same time the pump was. I was under the impression that a pump would or would not work, that there was really no in-between. What could cause a pump to only half work?
 

manicmechanic007

5 Year Member
Sep 26, 2017
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Roy, Utah
Jun 23, 2022
#9
  • Jun 23, 2022
  • #9
The new pump may have a bad valve or o ring and loose pressure
I would assume your pump is okay and your gauge is sticky or slow to read
And that the car has something else going on to cause the drivability concern
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
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polk county florida
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#10
  • Jun 23, 2022
  • #10
Did you change the filter?
 

Altered

New Member
Apr 24, 2021
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Iowa City
Jun 23, 2022
#11
  • Jun 23, 2022
  • #11
I haven't gotten around to it yet, probably won't be until Saturday.
 

Altered

New Member
Apr 24, 2021
10
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Iowa City
Jun 25, 2022
#12
  • Jun 25, 2022
  • #12
Update: Using a different fuel pressure test gage with the "old" filter, I got immediate and consistent reading of 30psi at idle with vacuum hose on and 38psi with it off. At this point, filter doesn't appear to be the issue. I changed it anyway just to be sure, and the readings with the new filter were identical. One thing I did notice is when the windows are up, and the vent is on, I can smell gas fumes. It's not very strong, and it goes away eventually.
I'm completely stumped at why the car feels like it has 10 to 15 less horsepower. As always, any help is extremely appreciated
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,864
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polk county florida
Jun 25, 2022
#13
  • Jun 25, 2022
  • #13
Is the charcoal canister hooked up?
 

Altered

New Member
Apr 24, 2021
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1
Iowa City
Jun 25, 2022
#14
  • Jun 25, 2022
  • #14
Yes it is. I did figure out the fumes issue. The car is still down on power.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,864
10,535
203
polk county florida
Jun 25, 2022
#15
  • Jun 25, 2022
  • #15
Check codes
 

Altered

New Member
Apr 24, 2021
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1
Iowa City
Jun 25, 2022
#16
  • Jun 25, 2022
  • #16
I did, there aren't any that relate to the motor or associated components
 
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