4.6 spitting o-rings out of 2 injectors

[skidad]

Confused. When do you get the 90psi? Explain the exact
Conditions

i drove 25 mins home from work (highway) - was watching the FRP and it jumped up 51 3x and thought it odd. Anyhow, I got home and realize if i turn the key back i thought i was only goin back 1 click but was going back 2 click and shutting off the scanner - so I moved it forward 1 click and it started reading fuel pressure again. It was like at 50 then kept slowly creeping up. I took stills and video of it - i was able to duplicate it twice by starting the engine again and watching it again.

I just got back from skiing and mustang was cooled down so I turned the key to the same exact spot but didn't start it and it read a constant 47-48 PSI. I started the engine and ran it 30 seconds and running 40 psi at idle and then turned the key back to just on and it stayed at 48 PSI again. so this is what I think is expected, BUT what I saw around lunch time was reading 90 psi. I figure that after it heat soaks a bit it's probably jumping to 100 psi and pushing that fuel rails up and orings out.

so what fuel pump is a good one or doesn't it matter the brand? I can hit up rockauto or just a parts store.

 

[highplace11]

Confused. When do you get the 90psi? Explain the exact
Conditions

Agreed to a point, BUT when he kills the ignition is when it' s happening on OBD so clearly just defaulting to #WHATEVER default MAX number...

Mark

 

[highplace11]

i drove 25 mins home from work (highway) - was watching the FRP and it jumped up 51 3x and thought it odd. Anyhow, I got home and realize if i turn the key back i thought i was only goin back 1 click but was going back 2 click and shutting off the scanner - so I moved it forward 1 click and it started reading fuel pressure again. It was like at 50 then kept slowly creeping up. I took stills and video of it - i was able to duplicate it twice by starting the engine again and watching it again.

I just got back from skiing and mustang was cooled down so I turned the key to the same exact spot but didn't start it and it read a constant 47-48 PSI. I started the engine and ran it 30 seconds and running 40 psi at idle and then turned the key back to just on and it stayed at 48 PSI again. so this is what I think is expected, BUT what I saw around lunch time was reading 90 psi. I figure that after it heat soaks a bit it's probably jumping to 100 psi and pushing that fuel rails up and orings out.

so what fuel pump is a good one or doesn't it matter the brand? I can hit up rockauto or just a parts store.

THAT!!!... Peaking to 51psi is nothing to worry about, let's be honest here, blowing that rail off and BENDING THE RAIL HOLDS which was the OP would be sick pressure man, no way no how... Please replace your rail... Unless you have the incorrect length injectors, I mean bro some cars we run @OVER60-80PSI on E85 no problem EVER!!!...

Mark

 

[highplace11]

THAT!!!... Peaking to 51psi is nothing to worry about, let's be honest here, blowing that rail off and BENDING THE RAIL HOLDS which was the OP would be sick pressure man, no way no how... Please replace your rail... Unless you have the incorrect length injectors, I mean bro some cars we run @OVER60-80PSI on E85 no problem EVER!!!...

Mark

Don't start throwing parts at it that won't change anything (FUEL pressure and FUEL PUMP IS DOING EXACTLY WHAT IT IS SUPPOSE TO)... Either buy the eBay rail or preferably buy a good upgraded one and call it a day, no fuel pressure is going to BLOW that thing off unless you have a different issue, you showed us a photo of a BENT rail... I agree hypothetically bending it back would be OK but those fukers' are tough metal too, maybe it has a bad connection/weld where the bracket connects to the actual rail, dig in deeper...

Mark

 

[skidad]

THAT!!!... Peaking to 51psi is nothing to worry about, let's be honest here, blowing that rail off and BENDING THE RAIL HOLDS which was the OP would be sick pressure man, no way no how... Please replace your rail... Unless you have the incorrect length injectors, I mean bro some cars we run @OVER60-80PSI on E85 no problem EVER!!!...

Mark

i don't know, maybe your right but i will verify it tomorrow afternoon once I get my analog gauge adapter.

 

[highplace11]

https://www.americanmuscle.com/bbk-fuelrail-9904.html or the eBay used option or look/search more options, depends on if you like custom stuff now is a good time to start, I'm out but will follow, we like seeing problems solved.... Wish I could see that car....

Mark

 

[highplace11]

i don't know, maybe your right but i will verify it tomorrow afternoon once I get my analog gauge adapter.

Yes sir!!!... That car wouldn't run properly if the fueling wasn't right, there really is no such thing as a random huge spike in those scenarios....

Mark

 

[gkomo]

Don’t people with FMUs see super high rail pressures? Just thinking out loud.

 

[skidad]

so i was able to duplicate the 90 PSI with a analog gauge. I took the car for good ride, shut it off and hooked up the analog gauge. Then ran it a few minutes and shut it off and took out the key - the gauge read 40 psi as soon as I shut it off. i watched it slowly rise from 40 to 91 - it took 10 mins

PS - i had to replace the Schradar valve/port to get my gauge to work. Old one had too short of a pin.
PPS - my analog gauge exactly matches whatever the scanner says.

Doesn't this point to the fuel pump check valve? I think I've ruled out the FRPS, right?

 

[Noobz347]

Right

 

[Mustang5L5]

Yup. Mine does not do that. Key off, pressure drops slowly

 

[skidad]

guys, do you think it's my fuel pump or do I need to check anything else? Asking b/c it looks like a 'fun' project

 

[squeak93]

I would lean towards a check ball valve issue. It certainly should not be building up. I would be dropping the tank to start checking things for sure.

 

[skidad]

so i pulled the tank today and put a new pump in. The old one was a Delphi FG0827 (with a 2024 date code) and the one ordered was the same Delphi pump. It didn't fix the problem - I'm pretty confused right now what else it could be unless it just doesn't like these Delphi pumps.

 

[squeak93]

Very interesting.

Pressure still increases AFTER engine/key are off, correct?

For fun - I ran this through GPT. Use any GPT advice with caution BUT :

The Delphi replacement pump for the 1999–2004 Mustang GT is notorious in the Ford community for having an over-engineered check valve that lacks the proper high-pressure bleed-off (PPRV) threshold. While Delphi makes excellent electronics, their mechanical returnless fuel pump modules for this specific Ford generation behave like a completely locked hydraulic dead-end when the key is turned off.

Why the Delphi Pump is Twisting Your Fuel Rails
When your car shuts down, heat soak from the 4.6L V8 transfers directly into the metal fuel rails. The trapped fuel expands.

• The Factory Pump: Designed to crack its pressure relief valve open if heat expansion pushes pressure past ~60 PSI, safely venting it into the tank.
• The Delphi Pump: Does not bleed off. It locks up tighter than a hydraulic jack, forcing the pressure to 90–100+ PSI until something physically gives way—which, in your case, is the metal injector hold-down tabs and the rails lifting.

Your Actionable Fixes
Because you have confirmed the pump brand, you have two direct paths to permanently fix this and protect your fuel system from mechanical damage:

1. Modify the Current Pump Assembly (PPRV Delete):
   If you do not want to buy another pump, you can pull the Delphi unit back out of the tank. Inside the pump hat, you will find the plastic PPRV (Positive Pressure Relief Valve) assembly. You can completely bypass this valve by replacing it with a short section of submersible fuel line hose (specifically rated SAE 30R10, such as Gates Submersible Fuel Line Hose) and stainless steel fuel injection clamps. Removing the restrictive Delphi valve completely eliminates the physical lock, preventing any future hydraulic pressure spikes.
   
   • Note: The only side effect is that fuel pressure will bleed off normally when parked. You will simply need to turn the key to the "ON" position for two seconds to prime the system before cranking the engine.
2. Swap to an OEM-Spec Pump:
   Return the Delphi pump and replace it with a genuine Motorcraft unit or a high-performance drop-in assembly from brands like Walbro / TI Automotive. These brands use internal check-valve architectures that safely release thermal expansion pressure back into the tank without locking up.

 

[skidad]

yes with key off - yes that is very interesting - now wishing i didn't cheap out on the pump or at least pulled the pump first.