• Mustang Forums
  • 1996 - 2004 SN95 Mustang -General/Talk-
  • SN95 4.6L Mustang Tech

Fuel Pressure never changes...

  • Thread starter Thread starter RoadconeTuning
  • Start date Start date Jun 16, 2008
R

RoadconeTuning

New Member
Apr 4, 2006
458
0
0
Upstate, SC
Jun 16, 2008
#1
  • Jun 16, 2008
  • #1
well... this has bothered me for a long time... my fuel pressure stays at 40psi according to my predator... idle, wot, decel... doesnt matter... the only time i see it change is during tip in... it'll dip to 38 or 39psi and then come back to 40psi instantly...

car has 66k miles on it, stock fuel system, just bolt-ons and the car SEEMS to run fine most of the time... when it is 80+ outside it feels like the car leans out over 3500rpm so i usually get out of it and start trying to pay attention to it...

another thing is sometimes when i start it ti'll go WAY lean about 5 seconds after i cranks... it stumbles and if im moving the car around when it happens and i roll into the gas it just falls on its face an then recovers about a second later...


is this FPDM related or fuel pump related or could it be a faulty pressure sensor on the fuel rail???

remember its just a bolt-on GT with nothing crazy like nitrous or a blower...
 

oxfordgt

Member
Apr 30, 2006
288
2
18
Clarksville, TN
Jun 17, 2008
#2
  • Jun 17, 2008
  • #2
Do you have your vacum connected to your FPS on the driverside fuel rail? At idle fuel pressure should be 30psi and go to 40psi under load. Tip in should jump up to around 43psi and settle back to 40psi. It sounds like you have no vacum refrence. You really need to fix that.
 

blkhorse

Founding Member
Oct 30, 1999
345
1
18
Jun 17, 2008
#3
  • Jun 17, 2008
  • #3
I don't know what pressure the predator is reading,but sounds like it is the delta pressure which the computer tries to keep it at 39 psi, so thats normal. datalog fuel pressure,maf counts,02's,iat's,temps etc when the problem occurs could help in finding problem
 
B

BobHyatt

New Member
Aug 7, 2007
378
1
0
Jun 17, 2008
#4
  • Jun 17, 2008
  • #4
RoadconeTuning said:
well... this has bothered me for a long time... my fuel pressure stays at 40psi according to my predator... idle, wot, decel... doesnt matter... the only time i see it change is during tip in... it'll dip to 38 or 39psi and then come back to 40psi instantly...

car has 66k miles on it, stock fuel system, just bolt-ons and the car SEEMS to run fine most of the time... when it is 80+ outside it feels like the car leans out over 3500rpm so i usually get out of it and start trying to pay attention to it...

another thing is sometimes when i start it ti'll go WAY lean about 5 seconds after i cranks... it stumbles and if im moving the car around when it happens and i roll into the gas it just falls on its face an then recovers about a second later...


is this FPDM related or fuel pump related or could it be a faulty pressure sensor on the fuel rail???

remember its just a bolt-on GT with nothing crazy like nitrous or a blower...
Click to expand...

Forget that reading and hook a gauge to the valve / connector on the fuel rail and measure the pressure _there_. If you unhook the vacuum to the regulator the pressure ought to increase. The exact values depend on your vehicle specifics, but this is the way to check fuel pressure, not thru the computer...
 

oxfordgt

Member
Apr 30, 2006
288
2
18
Clarksville, TN
Jun 17, 2008
#5
  • Jun 17, 2008
  • #5
He has an 04. Not as simple to just hook a gauge up you have to buy an adapter that goes under the FPS. 99-04 don't have a regulator you can get to in the engine bay.

OP how do you know it is lean after start up causing stumble. It is more then likely a rich condition which is causing it to stumble and fall on its face again pointing to no vacum refrence for the FPS.
 
R

RoadconeTuning

New Member
Apr 4, 2006
458
0
0
Upstate, SC
Jun 17, 2008
#6
  • Jun 17, 2008
  • #6
AEM Wideband indicates that it goes over 18:1 and shows "--"

and you are correct in me not having a valve to check fuel pressure with otherwise i would have since i have an 04...


the pressure delta sounds right and is originally what i thought... and i DO have the vacuum line connected to the sensor that sits on the drivers side fuel rail... that is the first thing i checked actually...

ill dig thru the predator datalogs with my laptop and see if i can find actual fuel pressure and not injector deltas....

if you think of anything else please post up.. ive been trying to figure this out for months lol....
 
You must log in or register to reply here.

Similar threads

B
'03 GT Fuel Pump/Pressure/Delivery Issue
  • Brodach
  • Oct 21, 2025
  • SN95 4.6L Mustang Tech
Replies
8
Views
928
SN95 4.6L Mustang Tech Oct 31, 2025
gkomo
I
1994 Cobra - Fuel Pressure Drops Revving at Idle
  • IconicSilverGT500
  • Apr 11, 2026
  • 1994 - 1995 Specific Tech
Replies
16
Views
475
1994 - 1995 Specific Tech Apr 24, 2026
Mcmahst
F
Trouble Shooting Sudden Shift of AFR at low RPM
  • Farva_84
  • Jun 6, 2025
  • SN95 4.6L Mustang Tech
Replies
8
Views
507
SN95 4.6L Mustang Tech Jun 8, 2025
Farva_84
F
T
Help with a 95 5L crank no start
  • TheBubbaJoe
  • Nov 4, 2025
  • 1994 - 1995 Specific Tech
Replies
3
Views
461
1994 - 1995 Specific Tech Nov 5, 2025
AeroCoupe
1999 Mustang GT - No Start- Need help understanding the fuel pump circuit and how it works
  • 99gtGuy
  • Nov 4, 2025
  • 1996 - 2004 SN95 Mustang -General/Talk-
Replies
1
Views
536
1996 - 2004 SN95 Mustang -General/Talk- Nov 7, 2025
04newedge-
0
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1996 - 2004 SN95 Mustang -General/Talk-
  • SN95 4.6L Mustang Tech
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?