Mustang5L5’s progress thread - Rear window fun

Noobz347 · Nov 15, 2025

I just had a thought but store it away for later:

I have only ever seen this once before and it was on an earlier model year than yours.

You may have a return line restriction that is giving you a false FP reading at idle. What I mean is that when the injectors [do] open up, they consume enough fuel volume that your regulator [becomes] the bottleneck vs. whatever might be downstream.

It would be semi-simple to check by disconnecting the return line at the rail and running a length of hose into a fuel can.

Not of course, until after you get to the end of checking out your fuel inlet suspicions.

I mean... even a partially pinched return line can cause something like this where the pinch is acting like an FMU but without boost reference.

It came to me as the result of another discussion with @from6to8 this morning.

Mustang5L5 · Nov 19, 2025

Well, grabbed another pressure sensor off Amazon and made this little grenade from some parts at work.

Hooked it up to the car and ran it up and down to verify pressure and the MS is reading spot on, so i trust the numbers. TBH i wanted to do this anyway just to be sure as i'm basing my fuel injector data off this value.

Just for the hell of it, i ran the fuel pump with the engine off by commanding it on with Tuner Studio. No vac since engine is off. Ran right at 29-30psi. Aww damn. Pretty sure I verified this with engine running but I could have sworn I set this with pressure at 39psi with engine running. I'm 99.5% sure I did. This is an interesting discrepancy. Was too late to fire up the engine so maybe i'll play with this on the weekend. I need to adjust this with engine running anyway as my pressure should be a little higher with the pump at 14.4 volts vs the 12V or so I have with the engine off.

Trogdor · Nov 19, 2025

Mustang5L5 said:
Well, grabbed another pressure sensor off Amazon and made this little grenade from some parts at work.

Hooked it up to the car and ran it up and down to verify pressure and the MS is reading spot on, so i trust the numbers. TBH i wanted to do this anyway just to be sure as i'm basing my fuel injector data off this value.

Just for the hell of it, i ran the fuel pump with the engine off by commanding it on with Tuner Studio. No vac since engine is off. Ran right at 29-30psi. Aww damn. Pretty sure I verified this with engine running but I could have sworn I set this with pressure at 39psi with engine running. I'm 99.5% sure I did. This is an interesting discrepancy. Was too late to fire up the engine so maybe i'll play with this on the weekend. I need to adjust this with engine running anyway as my pressure should be a little higher with the pump at 14.4 volts vs the 12V or so I have with the engine off.

Any way to regulate the voltage to 14.4v using a higher voltage battery or a Kenny Bell Boost-a-Pump?

Mustang5L5 · Nov 20, 2025

Trogdor said:
Any way to regulate the voltage to 14.4v using a higher voltage battery or a Kenny Bell Boost-a-Pump?

At this point, i'm trying to not over complicate this and just do simple things. Just going to fire u the car next and ensure i didn't do something stupid when I set the fuel pressure and go from there.

AeroCoupe · Nov 20, 2025

Man I hate to say it but if that is all it is I would take it and just beat myself up with a cold adult beverage.

Mustang5L5 · Nov 20, 2025

AeroCoupe said:
Man I hate to say it but if that is all it is I would take it and just beat myself up with a cold adult beverage.

Many have been consumed.

It's in the 20s now with daytime high of 40s, so motivation to worn on car is waning. However I need to know so i'll mess with it this weekend with beer in hand.

I think i might order a new diaphram for the FPR anyway. Couldn't hurt given the age of the unit. There is still a few things in the back of my mind that i saw that still lead me to think it won't be this easy.

Kid wita 5oh · Nov 20, 2025

Just a thought...have you check voltage AT the pump (not just in the circuit) while it's running?...are you still running the stock wiring to the pump or have you already upgraded to a big gauge wire with a relay?

AeroCoupe · Nov 20, 2025

Mine failed years ago and I got a Aeromotive. Just a couple years ago I ordered a diaphragm kit for the Kirban and rebuilt it for a spare.

FastDriver · Nov 20, 2025

I'd check fuel filter flow direction before paying for AFPR fixes.

Mustang5L5 · Nov 21, 2025

Well, fired it up for a quick second just to check.

Had to readjust the pressure due to voltage. I set it at 12v but at 14.4v the pressure was 47psi vac off. So had to lower it down to 39 psi vac off.

So, 39 psi vac off, 32psi with vac on.

Quick blip of the throttle…drops to 29psi.

So something is wrong.

FastDriver · Nov 21, 2025

Yep. Definitely wrong. Should go up on throttle blip. Not sure that narrows it down. Doesn't eliminate fuel pump or hose, but my instinct is with yours at the AFPR.

Not sure this will mean anything either, but what happens with throttle blip when you don't have the vacuum on it?

Mustang5L5 · Nov 21, 2025

FastDriver said:
but what happens with throttle blip when you don't have the vacuum on it?

Good question. I should try that and see what it does.

Ordered up a new fuel filter. Going to swap it and cut open the old one just to see

Kid wita 5oh · Nov 21, 2025

I'm gonna say it again...and there's a 99 % chance I'm wrong ... But ....if you have that big of a difference in pressure from battery voltage to alternator voltage... Have you upgraded the wiring to the pump....?

WhiteCobra95 · Nov 22, 2025

I'm thinking it's the regulator itself. It shouldn't be that sensitive to pump voltage at idle (12 vs. 14). Even if the voltage goes up and the pump flow increases, the regulator should just open up more to relieve the flow and compensate the pressure. The flow difference from a 2V jump should be well within the regulator's response curve, especially at idle with a constant pressure (ambient) on the reference port. The regulator spring and/or diaphragm are probably warn out. If you have one on hand, I suggest swapping a stock style / fixed regulator to compare the behavior. (This is where I start questioning the performance and quality of aftermarket parts.)

Good luck! Very frustrating!

Mustang5L5 · Nov 22, 2025

Here's some more data. Did another start

Key cycle on: 29psi. Unable to get any higher no matter how many times I cycle the key.
Cranking: 29psi
Engine running(vac line on) 32 psi
Engine running(vac line plugged) 39psi
Reving engine (Vac line on) 29 psi
Revving engine (vac line plugged) 30 psi

The unable to go higher than 29 psi during initial prime is telling. I wonder if i have a split S-hose that can hold up to 29psi

Trogdor · Nov 22, 2025

Do you have a stock regulator that you could test with?

AeroCoupe · Nov 23, 2025

Just me but I would put a diaphragm kit in the Kirban before I would drop the tank. Especially if you are putting the car up for the winter and have a full tank of stabilized gas in it.

Noobz347 · Nov 23, 2025

The FPR has [no capacity] to reduce the fuel pressure as vacuum is reduced.

Period

Mustang5L5 · Nov 23, 2025

I don’t have a stock regulator kicking around. I’ll see if any locals have one I can grab for the sake of troubleshooting. I could buy an Amazon china one for $15 and swap it on just to see what happens.

What’s interesting is during key cycle, it pegs to 29psi and no more, and immediately drops to 22-23 when the pump kicks off. I clearly remember when I had a mechanical guage on this car that it pegged instantly to 39psi on the first key cycle.

I might try a cheap parts store reg. If I see the same pattern, than that rules it out.

Mustang5L5 · Nov 23, 2025

dug into it to pull off the reg

Nothing jumps out at me as bad. A little oxidation on the vacuum side, but no rips.

Got a cheapo stock reg coming tomorrow. I’ll throw it on and see if key cycling and running the pump results in 39psi.

If that checks out, I’ll order up a diaphragm for this, and put this all back together and move onto the tank side of things.

Upper intake was soaked in fuel, but I see no signs of fuel on the vac side of the regulator. Something to pay attention to. Might just be a tuning thing.

Mustang5L5’s progress thread - Rear window fun

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Mustang5L5

That is…until I whipped out my Bissell

Trogdor

Mustang5L5

That is…until I whipped out my Bissell

AeroCoupe

lube between the nut and the face. I know my lubes

Mustang5L5

That is…until I whipped out my Bissell

Kid wita 5oh

I'm definitely not in the original hole

AeroCoupe

lube between the nut and the face. I know my lubes

FastDriver

I was uncomfortably high & wearing a helmet

Mustang5L5

That is…until I whipped out my Bissell

FastDriver

I was uncomfortably high & wearing a helmet

Mustang5L5

That is…until I whipped out my Bissell

Kid wita 5oh

I'm definitely not in the original hole

WhiteCobra95

Mustang5L5

That is…until I whipped out my Bissell

Trogdor

AeroCoupe

lube between the nut and the face. I know my lubes

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

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Mustang5L5’s progress thread - Rear window fun

Stangnet Facilities Maint Tech... Er... Janitor

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

lube between the nut and the face. I know my lubes

That is…until I whipped out my Bissell

I'm definitely not in the original hole

lube between the nut and the face. I know my lubes

I was uncomfortably high & wearing a helmet

That is…until I whipped out my Bissell

I was uncomfortably high & wearing a helmet

That is…until I whipped out my Bissell

I'm definitely not in the original hole

That is…until I whipped out my Bissell

lube between the nut and the face. I know my lubes

Stangnet Facilities Maint Tech... Er... Janitor

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

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