Mustang5L5’s progress thread - Rear window fun

nicholase · Nov 23, 2025

Not sure if this is helpful or not. It's just part of the instructions that came with my tuning download.

It sounds like a bad pump or S- hose to me. But I wonder if the intake being soaked in fuel is a clue. Like a bleeding down injector or something.

Mustang5L5 · Nov 23, 2025

It’s evenly soaked, but could be because I was adding fuel to the tune because I didn’t realize I was losing 10psi of pressure during some instances. For the moment, going to shelf that issue for now.

I do think it’s S-hose related. Just a hunch. But wanted to rule out the reg first before I start siphoning the tank.

Kid wita 5oh · Nov 24, 2025

I'm sure I have a stock regulator kicking around if you want to try that route

FastDriver · Nov 24, 2025

Aside from fuel coming through the regulator, why would the upper be soaked with fuel? That's not right... The injectors fire at the back of the valves at the base of the lower intake, and the fuel should get pushed through the combustion cycle. Hmm...

Still, my gut agrees with Noobz. At idle, there's not enough draw to cause the fuel pressure to drop on a blip. I'm not ruling it out either, but I can't visualize how decreasing vacuum could make the regulator cause a decrease in fuel pressure. So, yeah... definitely leaning towards a pump, hose, clogged/crimped fuel line or filter.

Still... scratching my head as to why 29 on blips and also 29 under load. One's using substantially more fuel than the other...

Ugh... we've over-thought this. Time to get your hands dirty.

AeroCoupe · Nov 24, 2025

It could be that the regulator is just not reacting to fuel demand. Blip the throttle more volume is needed and the regulator isn’t allowing more fuel to flow i.e. the rails don’t have the volume. I know that is a stretch and the more you guys talked about it I am also leaning towards a fuel delivery issue other than the regulator but it’s easy to rule out vs dropping the tank.

Mustang5L5 · Nov 24, 2025

FastDriver said:
Aside from fuel coming through the regulator, why would the upper be soaked with fuel? That's not right... The injectors fire at the back of the valves at the base of the lower intake, and the fuel should get pushed through the combustion cycle. Hmm...

I left out some info. I cold started the car right before I pulled the intake. I had to move it around int he garage so fired it up, moved it, shut it down. Car was 45 degrees when i fired it up and barely made it past 80. ALso, i'm a tad rich during warmup. Actually...i'm way too rich at 45 degree startup. Remember, when I was tuning it was in the 70's. Now it's colder so I need to pull some fuel out from my WUE it seems. I parked car and immediately pulled intake. DUring a cold start like this i would expect to find fuel. If i pulled it off while fully warmed up i would guarantee it would be dry.

Long story short....lets ignore this for now.

Anyway, i got my hands on a stock regulator. Going to swap it on and see if anything changes. Hold on to your butts.

Mustang5L5 · Nov 24, 2025

AeroCoupe said:
It could be that the regulator is just not reacting to fuel demand. Blip the throttle more volume is needed and the regulator isn’t allowing more fuel to flow i.e. the rails don’t have the volume. I know that is a stretch and the more you guys talked about it I am also leaning towards a fuel delivery issue other than the regulator but it’s easy to rule out vs dropping the tank.

The blipping test is easier to do than driving the car right now. I have datalogs from driving that car showing it does the same thing. Essentially i cruise at 34psi and then when i roll onto the throttle or go WOT it drops to 29psi. Any load on the engine drops the FP. It does the same thing when I blip the throttle. I would assume less fuel demand during throttle blips in neutral, but i see a drop from 34 to 29 even during a quick no-load throttle stab.

Anyway, stock reg is on the car. Need to put the intake back on as i'll need to start engine to really test it. If i can bang this out in the next 10 mins I can fire it up real quick before it gets too late.

FastDriver · Nov 24, 2025

Mustang5L5 said:
Actually...i'm way too rich at 45 degree startup. Remember, when I was tuning it was in the 70's. Now it's colder so I need to pull some fuel out from my WUE it seems.

And this is why I so encourage folks to learn to tune. Even incredible tuners are not going to see your car in all the conditions you will. Plus, it just ups your game a couple of levels to start to understand all of the variables and adjustments at play.

Mustang5L5 · Nov 24, 2025

Mustang5L5 said:
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

Ok. Take this and digest on it. The above was what happened when using the Kirban AFPR

Key on, engine off prime. 45PSI
Key on, engine off, when prime stops: 29 psi (odd coincidence eh?)

Engine running, VAC line OFF: 52PSI!
Engine running, VAC line OFF, REV: 47PSI

Engine Running, VAC line ON: 47PSI
Engine Running, VAC line ON, REV: 45psi.

I don't think the Kirban AFPR is the issue here. This FPR is non-adjustable, but even though the pressures are different, im seeing the same behavior. It's kinda of a coincidence that when the prime cycle stops, the pressure drops to 29psi. That number seems to be frequently seen here. I'm really starting to lean towards S-hose issue.

Also, don't buy $15 cheapo no-name FPR's off Amazon. lol I'm returning this POS but now I don't feel bad about it. Sucker is WAY off and throws off my entire idle tune.

FastDriver · Nov 24, 2025

seems like it rules out the FPR. If you have double-tapped the filter, then time to go in-tank.

nicholase · Nov 24, 2025

When you're in there consider extending the return line to the bottom of the tank like the aeromotive hanger.

Mustang5L5 · Nov 24, 2025

Oh boy. Let’s see it this turns into me revamping my entire fuel system with 6AN line all the way to the engine bay.

Mustang5L5 · Nov 25, 2025

lol. Let’s fix the problem first.

Pulled the old fuel filter. Cut open the filter. Didn’t really see anything bad about it

Started siphoning off the gas. It’s perfectly clean

Mustang5L5 · Nov 26, 2025

Siphoned off about 8-9 gallons and dropped the tank.

No rust inside.

Pump looked fine. No split s-hose from what I could see

This was the bottom of the filler sock. Would this be enough to do it?

Mustang5L5 · Nov 26, 2025

How bout that rear suspension?

Btw harbor freight trans jack also works very well for gas tanks.

FastDriver · Nov 26, 2025

I guess if you wanted to know the answer to that, you'd have to flow test it with and without the sock. But, I'm sorry that there's no clear Smoking Gun. Kind of sucks to play guessing games when a pump is expensive. I like having spare parts on the shelf for stuff like this. I could swap my known good pump and see if it cleared things up. If interested, I'd send it to you and you can give it a shot. My fuel system box is pump, regulator, injectors you have pretty much everything except the rails, lines, and filter at that point.

RaggedGT · Nov 26, 2025

Isn’t that a new tank?

Mustang5L5 · Nov 26, 2025

RaggedGT said:
Isn’t that a new tank?

Relatively new. 8 years?

i have no idea what that crud stuck to the bottom of the sock is from.

Mustang5L5 · Nov 26, 2025

So did a fuel flow test. Hooked everything back up except the return and stuck that into a gallon gas can.

Idea was to run the system at 12v and calculate how much flow flow I have in 60 seconds and compare to this chart.

https://www.tanksinc.com/pdf/2_series_fuel_pump.pdf

So my test was 12v at 44psi. I got just about 1/2 gallon in 60 seconds.

According to this chart I should be around 0.66-0.7 gallons after 60 seconds. I got 0.5ish.

I need to check voltage under load, but it's looking like this might be a pump issue.

Mustang5L5 · Nov 26, 2025

FastDriver said:
I guess if you wanted to know the answer to that, you'd have to flow test it with and without the sock. But, I'm sorry that there's no clear Smoking Gun. Kind of sucks to play guessing games when a pump is expensive. I like having spare parts on the shelf for stuff like this. I could swap my known good pump and see if it cleared things up. If interested, I'd send it to you and you can give it a shot. My fuel system box is pump, regulator, injectors you have pretty much everything except the rails, lines, and filter at that point.

WHat size pump is that? Might be worth testing with it.

Got this is my cart but trying to not blindly buy stuff until i'm sure I need it

https://lmr.com/item/LRS-9350B/1986-97-Mustang-190-Lph-Walbro-Gss242-Fuel-Pump

But first, i need to do some voltage drop tests to rule that out

Mustang5L5’s progress thread - Rear window fun

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

That is…until I whipped out my Bissell

I'm definitely not in the original hole

I was uncomfortably high & wearing a helmet

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

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

I was uncomfortably high & wearing a helmet

That is…until I whipped out my Bissell

I was uncomfortably high & wearing a helmet

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

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

I was uncomfortably high & wearing a helmet

Been here over a DECADE and still no CT

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

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