Fuel Fox fuel pressure cold starts

Kingranch

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Nov 24, 2020
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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.
 
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MustangIIMatt

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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
Mod Dude
Aug 25, 2016
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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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Haste347

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Oct 6, 2020
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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?
 

Steve10608

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Mar 12, 2019
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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.
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?
 

MustangIIMatt

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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?
It would indicate a failing pump, partially clogged filter or screen, a failed regulator, or a leak.

Pump and regulator are the usual suspects.