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Fuel Fox fuel pressure cold starts

  • Thread starter Thread starter Kingranch
  • Start date Start date Nov 24, 2020
K

Kingranch

Member
Nov 24, 2020
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Austin
Nov 24, 2020
#1
  • Nov 24, 2020
  • #1
My 1990 lx 5.0 has trouble 1st in the morning to start it stumbles and will stall if I dont give it a tiny bit of gas.
Fuel pressure shows 40 psi primed and drops to 30 when car starts.

Funny thing is after it warms up for a second it runs great and restarts easily no stumble...only after it sits over 8 hrs.

I did notice if I unplug the vacuum on fpr it jumps to 40 and on a cold start in morning it does stumble but idles higher like its rich...

Could the for cause this early morning hard start condition? It's a ford stock fpr and holds good at 30 when running and it's not leaking that I can see or smell from the vac nipple.

Everything is stock as far as the fuel system goes.. 19lb injectors and new fuel pump and filter last year...also injectors were rebuilt.

I noticed the fuel pressure bleeds off overnight but from internet search this is fairly common.
 

Adieu

Easy there, this ain't a dating site.
20+ Year Stangneter
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Nov 24, 2020
#2
  • Nov 24, 2020
  • #2
This is sometimes the early warning of a fuel pump failure, or of a slow leak in the line inside the fuel tank. The pressure bleeds down overnight due to either failure, and then it doesn't prime correctly when the key is first turned. Subsequent attempts allow it to build enough pressure to overcome the issue, and once the system is running it has no problem maintaining.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
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polk county florida
Nov 24, 2020
#3
  • Nov 24, 2020
  • #3
On cold startup, try priming the fuel system a couple/three times.
Cycle the key on without starting two or three times, see if the 'cold start stumble'goes away.
 
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Potomus Pete

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#4
  • Nov 24, 2020
  • #4
Good idea Karthief gonna try that.
 

General karthief

wonder how much it would cost to ship you a pair
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polk county florida
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#5
  • Nov 24, 2020
  • #5
Potomus Pete said:
Good idea Karthief gonna try that.
Click to expand...
I have my moments
 
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Haste347

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Oct 6, 2020
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Houston
Nov 27, 2020
#6
  • Nov 27, 2020
  • #6
Make sure your TB is correctly set. Unplug the IAB valve and set your idle with the TB screw to around 500/600-ish (assuming manual), make sure your TPS voltage is ~.8 volts at idle, then plug the IAB valve back in and see if that helps.
Building on the above recommendations, what is your PSI with the initial fuel pump prime? Does it take multiple primes to get to 40 PSI?
 

Decipha

Active Member
Aug 22, 2007
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New Orleans, LA
Dec 1, 2020
#7
  • Dec 1, 2020
  • #7
you have to also temporarily unplug the spout and tps plug along with the iac to set the throttle stop correctly. details- http://www.efidynotuning.com/idleair.htm

tps voltage is irrelevant

On all my custom tunes I set the fuel pump prime time from 1 sec (stock) to 3 seconds to build pressure.

No one asked but is this vehicle all stock?
 
S

Steve10608

Member
Mar 12, 2019
15
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13
Texas
Feb 25, 2021
#8
  • Feb 25, 2021
  • #8
General karthief said:
On cold startup, try priming the fuel system a couple/three times.
Cycle the key on without starting two or three times, see if the 'cold start stumble'goes away.
Click to expand...
Sorry about jumping in here. I'm having the same problem with my 95 cobra. Would having to prime the system two or three time indicate a failing fuel pump?
 

Adieu

Easy there, this ain't a dating site.
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#9
  • Feb 25, 2021
  • #9
Steve10608 said:
Sorry about jumping in here. I'm having the same problem with my 95 cobra. Would having to prime the system two or three time indicate a failing fuel pump?
Click to expand...
It would indicate a failing pump, partially clogged filter or screen, a failed regulator, or a leak.

Pump and regulator are the usual suspects.
 
S

Steve10608

Member
Mar 12, 2019
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Texas
Feb 25, 2021
#10
  • Feb 25, 2021
  • #10
MustangIIMatt said:
It would indicate a failing pump, partially clogged filter or screen, a failed regulator, or a leak.

Pump and regulator are the usual suspects.
Click to expand...
I have already replaced the regulator. Any suggestion on a good aftermarket fuel pump?
 

Adieu

Easy there, this ain't a dating site.
20+ Year Stangneter
Mar 7, 2002
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#11
  • Feb 25, 2021
  • #11
Steve10608 said:
I have already replaced the regulator. Any suggestion on a good aftermarket fuel pump?
Click to expand...
Walbro makes a good one. Avoid Airtex like the plague.
 
S

Steve10608

Member
Mar 12, 2019
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2
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Texas
Feb 25, 2021
#12
  • Feb 25, 2021
  • #12
MustangIIMatt said:
Walbro makes a good one. Avoid Airtex like the plague.
Click to expand...
Thank You......
 
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