Mustang5L5’s progress thread - Rear window fun

Mustang5L5 · May 28, 2026

AeroCoupe said:

For the hell of it I dumped all my symptoms and everything I did into Copilot and just wanted to see what it came back with. I gave it a pretty detailed/extensive list of symptoms and what i've done.

It said injectors.

Thing is i can't run the engine unless I get adapters since i changed my injector harness from EV1 to USCAR connectors. But my test is the key prime test. It will NOT prime past 29psi. If this line of thinking is correct that's the pressure one of the injectors must be leaking at. I think if i just swap the injectors, a key prime to 39psi will be VERY telling. The injectors are ahead of the regulator, and the system acts like i have a leak.

Forgot to mention. I changed the oil and it had gas in it. Also, the last few times i've pulled the intake, very wet with gas. Now, i looked past that because I've been doing a lot of cold starts to check the fuel pressure without fully warming the engine up...but I am more suspicious now..especially since I only have 1 wideband installed on my pass side.

Didn't swap yet. It was a beautiful night out last night and I took the kids out in the convertible for Ice Cream. It was absolutely perfect top-down weather. Going to rain all day Sat.

Single 47# injector is part number BR3Z-9F593-A. $60 plus shipping from Tasca. Nothing would make me happier than to place an order for that part.

2000xp8 · May 28, 2026

I don't know all that much about leaking injectors, never had one (i have had a #39 stick, but tapping it with the back end of a screwdriver fixed that).

If it were leaking, would it leak enough to never let it get past 29psi?
I would think it would go past that, then bleed and if it was leaking, why stop at 29 every time?

FastDriver · May 28, 2026

The thought crossed my mind there, but didn't you say this was happening with 19s? Also, hard to imagine a single stuck injector overwhelming a 190 LPH pump, even a 47, and what kind of crap parts would they have to be to have 3+ sticking?

Honestly, as thoroughly as you've gone through things, it very well could be a combination of things contributing. I'm not ruling out an injector problem, but it would surprise me if it was the only problem that sucked the pump dry & caused a fuel pressure drop.

I'm going back to a regulator issue. Torn diaphragm, despite your efforts. Did you smell the vacuum line to see if it reeks of gas? Introducing fuel there absolutely sucks fuel back through the intake & causes pressure problems.

gkomo · May 28, 2026

Where did you measure resistance when checking the wiring? What about something going on with the tank harness? Getting good voltage/resistance to the end of the body harness, but something going on from that connection to the actual pump hangar?

FastDriver · May 28, 2026

gkomo said:
Where did you measure resistance when checking the wiring? What about something going on with the tank harness? Getting good voltage/resistance to the end of the body harness, but something going on from that connection to the actual pump hangar?

That happened to me. Pump died and turned out it was mechanically fine, but there was a short in the pump bracket wiring. Was obvious because when I applied 12v to the bracket plug, it wouldn't go, but directly to the pump did. Could it be some kind of short that's allowing partial voltage?

AeroCoupe · May 28, 2026

Sounds stupid but could it be the fuel pump sock restricting flow somehow like maybe getting partially sucked into the inlet?

nickyb · May 28, 2026

nickyb said:
Change the fuel pump to a 255lph, my walbro 255 has held up over many years now.
Its cheap and easy so go for it.

I'm telling you its gonna be the fuel pump.

AeroCoupe · May 28, 2026

Thing is he tried two different pumps so do you think they both have the same issue?

General karthief · May 28, 2026

Very interested in the outcome, JWOL I would think a 'stuck' injector should not effect fuel pressure but IDK, I'm kinda lean'n toward the fuel pump at this juncture.
But what do I know

Sparky714 · May 28, 2026

How hard would it be to disconnect the feed and return line at the fuel rail and temporarily plumb them into an external regulator? If it builds full pressure at prime, the problem is on the engine. If it still only builds to 29psi, it's somewhere in the lines/tank /pump. At least you could isolate it to one section or the other.

nickyb · May 28, 2026

AeroCoupe said:
Thing is he tried two different pumps so do you think they both have the same issue?

Well I got three bad distributors and would have never ever have thought it could happen,just saying.

gkomo · May 28, 2026

Did you have to do any wiring upgrade to the hangar when changing the pump out for the first time? When I put my 255lph pump in it required some minor wiring changes… maybe something got fubar’d with that so the problem is pump agnostic.

I’m far from an expert but something tells me it’s wiring related from the tank harness to the pump.

That or a butt connector got lodged in the S hose ( like what happened to LMR ) lol

2000xp8 · May 29, 2026

FastDriver said:
I'm going back to a regulator issue. Torn diaphragm, despite your efforts. Did you smell the vacuum line to see if it reeks of gas? Introducing fuel there absolutely sucks fuel back through the intake & causes pressure problems.

I can't help but think regulator too.
29psi just seems like a too consistent number to be much else.
The fuel system isn't very complex and even if there were an injector problem the 190lph pump and the regulator should be able to compensate by dumping less fuel back in the tank. Any NA 302 easily runs on a 155lph pump, so the 190 would be more than enough to add the lost psi.
And if there was an injector problem, where exactly would all that fuel be going that you aren't aware of?

Mustang5L5 · May 29, 2026

2000xp8 said:
And if there was an injector problem, where exactly would all that fuel be going that you aren't aware of?

I just changed the oil, and it smelled gassy. It could be from all the cold/starts and not fully warming up while i sort though this. It also smells rich in the garage, but my AFR at idle is only 13.8 because any leaner get lopey. FOund some fuel in the intake as well when I pulled it. Coud be just tune related..but i can't put this out of mind.

AeroCoupe said:
Sounds stupid but could it be the fuel pump sock restricting flow somehow like maybe getting partially sucked into the inlet?

I replaced the sock with a different one. No change. If i drop the tank again i will remove it completely and see what happens

gkomo said:
Where did you measure resistance when checking the wiring? What about something going on with the tank harness? Getting good voltage/resistance to the end of the body harness, but something going on from that connection to the actual pump hangar?

original fuel pump wiring is not being used. I ran brand new 10G wire from the battery to the fuel pump through the hanger. The stock wiring is only activating the relay. zero change.

FastDriver said:
I'm going back to a regulator issue. Torn diaphragm, despite your efforts. Did you smell the vacuum line to see if it reeks of gas? Introducing fuel there absolutely sucks fuel back through the intake & causes pressure problems.

No gas smell in the reg. I've tried a few regs. Swapped a stocker on. No change. Bought a brand new kirban diaphragm. No change.

------------

I almost have the injectors swapped out. Lets just check this off the list and then we can circle back to revisiting some of the other items again.

Mustang5L5 · May 29, 2026

Swapped the injectors out.

No change.

By that i mean i see the exact same behavior when I key prime and run the pump. Key prime goes to the same pressure (29psi) and then drops and holds at 23.3psi. I've been seeing these numbers for months now so they are drilled into my mind what I expect to see when i do various steps. Zero change from the 47's to a spare set of 30's I have.

I think i'm going to take a step back and verify my pressure readings. "borrowing" a nice caibrated mechanical gauge from work and picked up some fittings to connect to the rail.

Also borrowing a vac hand pump to play with that to see if this is manifold pressure/vac related. my FPR is hooked to stock location so shouldn't be a prob. Using a hose and my lungs, i do see a response in pressure from the diaphragm when i suck/blow.

At this point i need to circle back to the basics and start over. Like someone said, these fuel systems are not that complicated. This should have been easy to find. Step #1 is confirm my pressure readings, so I'm going to do that (again).

Actually step 1a is to put the 47# injectors back in so i can run the engine

Kid wita 5oh · May 29, 2026

Ahh when you said gassy oil I would have guaranteed injectors... But not 2 sets exactly the same..

Not the pump either, that pump was flawless in my car..

Are you seeing the same psi on mech gauge and FP sensor (do you have a sensor on your set up?)

Mustang5L5 · May 29, 2026

Kid wita 5oh said:
Ahh when you said gassy oil I would have guaranteed injectors... But not 2 sets exactly the same..

Not the pump either, that pump was flawless in my car..

Are you seeing the same psi on mech gauge and FP sensor (do you have a sensor on your set up?)

The gassy oil is prob the tune. I’m a little rich at idle and have spent way too much time idling the car dealing with this. I was working on leaning this out when I found my issue

I've tried a mech gauge and a few transducers. They all show the same. I have another mech gauge I'm going to set up with a tee so i can run the transducer and gauge together

I’m pretty much starting over and starting with verifying the problem exists. Also going to try a different vac source even though I’m using the stock location port

Mustang5L5 · May 29, 2026

Found this in my logs. I want to put this here just so i have a record of it and it doesn't get lost.

This was with the vac line off and capped. So fuel pressure was static 39psi

For those not familiar with megalogviewer, the blue verticle line is the snapshot in time and to the right are the numbers at that moment and the corresponding colors.

So yellow is my TPS. Essentially when i hit the gas for a rev. You can see MAP immediately follows (0.1s behind the TPS hit), so no funky things happening there. White is my FP. I'm sitting at 39psi and then the rev comes and it drops to 30.1psi. It then recovers. You can see my RPM dips, and my AFR goes lean. Not shown is injector pulse width. It goes from 3ms to 14ms during the blip, so it's trying to add fuel.

nicholase · May 29, 2026

@Mustang5L5 I have a theory that could explain all of it. Been thinking about this for a while. Can you post a screen shot of your fuel pressure sensor set up in your software. You should have a screen with units and sensor types. Open them up to show the choices and what you have selected.

Edit- Also a pic of your FP transducer

Mustang5L5 · May 30, 2026

nicholase said:
@Mustang5L5 I have a theory that could explain all of it. Been thinking about this for a while. Can you post a screen shot of your fuel pressure sensor set up in your software. You should have a screen with units and sensor types. Open them up to show the choices and what you have selected.

Edit- Also a pic of your FP transducer

Here you go.

This is the exact main transducer i've been using.

LDM 0-100 PSI Pressure Sensor

LDM Pressure Sensor 0-100 PSI & 15ft cable with 90° boot combo; Fast-Response Pressure Sensor for fuel, oil, coolant, etc.

diyautotune.com

Mustang5L5’s progress thread - Rear window fun

Mustang5L5

That is…until I whipped out my Bissell

2000xp8

FastDriver

I was uncomfortably high & wearing a helmet

gkomo

now i can hopefully expect to receive the shaft

FastDriver

I was uncomfortably high & wearing a helmet

AeroCoupe

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

nickyb

I gotta say i never painted my nuts, Never Ever

AeroCoupe

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

General karthief

wonder how much it would cost to ship you a pair

Sparky714

nickyb

I gotta say i never painted my nuts, Never Ever

gkomo

now i can hopefully expect to receive the shaft

2000xp8

Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

Kid wita 5oh

I'm definitely not in the original hole

Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

nicholase

I lubed and pushed, until it was all the way in

Mustang5L5

That is…until I whipped out my Bissell

LDM 0-100 PSI Pressure Sensor

Similar threads

Mustang5L5’s progress thread - Rear window fun

That is…until I whipped out my Bissell

​

I was uncomfortably high & wearing a helmet

now i can hopefully expect to receive the shaft

I was uncomfortably high & wearing a helmet

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

I gotta say i never painted my nuts, Never Ever

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

wonder how much it would cost to ship you a pair

I gotta say i never painted my nuts, Never Ever

now i can hopefully expect to receive the shaft

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

I'm definitely not in the original hole

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

I lubed and pushed, until it was all the way in

That is…until I whipped out my Bissell

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