Mustang5L5’s progress thread - Rear window fun

2000xp8 · Jun 2, 2026

That's a pretty solid price on the accufab, especially when the aeromotive is $250+.

hoopty5.0 · Jun 2, 2026

FastDriver said:
Erm... I thought mine was an aero when you mentioned it, but it's an accufab. Can you send it back? Just tell em it didn't fit a stock setup. You might like this one more:

Damn! Thats a nice piece. Wish I would have known.

FastDriver · Jun 2, 2026

2000xp8 said:
That's a pretty solid price on the accufab, especially when the aeromotive is $250+.

That price was from my order email from maybe late '24. Bought from Lethal Performance and its $159, now:

Accufab 86-93 Mustang 5.0L Fuel Pressure Regulator

Accufab has developed this fuel pressure regulator to optimize your fuel pressure needs. The regulator is crafted from high quality billet aluminum and anodized black. Then, each is assembled using only top quality internal component parts. Also, each regulator is adjustable and boost sensitive...

www.lethalperformance.com

2000xp8 · Jun 2, 2026

FastDriver said:
That price was from my order email from maybe late '24. Bought from Lethal Performance and its $159, now:

Accufab 86-93 Mustang 5.0L Fuel Pressure Regulator

Accufab has developed this fuel pressure regulator to optimize your fuel pressure needs. The regulator is crafted from high quality billet aluminum and anodized black. Then, each is assembled using only top quality internal component parts. Also, each regulator is adjustable and boost sensitive...

www.lethalperformance.com

Regardless, it's still way cheaper.
Some companies just gouge on the prices just because they can, i guess that is aeromotive and pertronix...

Mustang5L5 · Jun 2, 2026

Well i actually did pretty good on the Aeromotive. Got it off Amazon for ~$200 from the Aeromotive store because I could get it next day.

Sold as new, arrived opened up (typical Amazon). I bitched at Amazon and they knocked $65ish off for sending me an opened part that someone had put teflon tape on the plug and vac port and sent back. So $140 was the damage. I was fine with that because there were zero signs of it actually being installed. No gas smell. No compression on the O-rings. Screws looked unused. No scratches. Had i seen any signs i would have sent it right back.

I will say, it's dead on with regards to holding pressure stable. With the kirban it was up and down 1psi or so in an oscillating manner. With the aeromotive, i set it to 39.2psi (because the 47# injector data I have is at that pressure) and it just held that. Pretty impressive.

But yeah...after the fact I kinda wish I went with the Accufab. I think for some reason I just didnt realize it had the port to install a gauge as well.

But yeah, need to start the engine and lets see if this fixes anything and then we will go from there. I got the intake back on late last night. I want to install my pressure gauge first and then we can fire it up and see.

Mustang5L5 · Jun 2, 2026

DIDNT MAKE A DIFFERENCE!

Argh. TBH is is frustrating. No change. 9psi loss in static (vac off plugged) pressure during a simple rev.

I also have about a 5 psi differential between the transducer mounted in the reg, to the mech guage at the stock location. Regardless though, I see the loss on both.

Mustang5L5 · Jun 2, 2026

Interesting observation after the fact.

Engine running. 44psi on transducer, 39psi on mech guage down by alt.

Engine off, run the pump: both read 36psi.

I wonder if i need to play with my voltage compensation in TS. Might have something to do with the different readings at 14.4v vs 12.4v.

I gotta play around with some things before i drop the tank. I'm just going to be testing various things out right now and might not update for a while

Kinda want to figure out a way to hang the pump in a jug of water safely. I want to remove the tank/baffle/sock from the equation

WhiteCobra95 · Jun 2, 2026

Maybe a dumb question that you've already covered: are you feeding the pressure transducer 5V and not 12V / system voltage? Have you double checked the stability of the 5V supply / is it really at 5V?

I went back to the post with a like to the transducer, and it shows 5V for the Vref. This should come from a 5V regulated sensor supply on the ECU. It should also use a conditioned sensor ground if possible (I don't know if the MS ECU has specific sensor grounds).

WhiteCobra95 · Jun 2, 2026

The sensor itself looks pretty robust in terms of it's range. It's even setup like a production automotive sensor with a functional span from 0.5 V to 4.5 V for short circuit detection. If MS supports diagnostics, you could set a short to ground fault threshold at 0.45 V and short to high at 4.6 V.

AeroCoupe · Jun 2, 2026

Mustang5L5 said:
Kinda want to figure out a way to hang the pump in a jug of water safely. I want to remove the tank/baffle/sock from the equation

Que? Are you going to test the pump in water? Water = density of 1.0 and gasoline = density of ~0.74. The viscosities are different (1.0 cP vs. ~0.6 cP) as well so the pump will not perform the same. I think you know this but just wanted to provide more filler here being you are going dark on us for a while

Mustang5L5 · Jun 2, 2026

Ha. Meant gas. Not water. Safe way of running gas outside of a tank without dropping too much coin.

Did some more testing and called it a night.

Engine off, pump running. Used a mityvac to apply vacuum. Pulled enough vac for pressure to drop from 38psi to 31psi. Released the vacuum.

Instant spike back to 38psi. No dip at all.

I gotta let this stew around a bit in my mind. That's essentially a vertical line back to full pressure. Am i chasing some sort of tuning/calibration issue?

Like said, going to play around with a few things first. But i wonder if I tore my entire fuel system apart over a tuning artifact? Thing is...car actually drives pretty darn good. If you asked me if i thought it was running a tad rich, i'd say yes. I need to check a few things, including my injector settings. I could have something way off which is causing injectors to open for much too long with the 47#ers.

I actually realized I can use the mityvac with the engine running. Would simulate no load release of vac to atm which should cause a pressure spike from 33-34 up to 39+. If that line is vertical, than it's gotta be my injectors dumping way too much fuel

Mustang5L5 · Jun 2, 2026

WhiteCobra95 said:
Maybe a dumb question that you've already covered: are you feeding the pressure transducer 5V and not 12V / system voltage? Have you double checked the stability of the 5V supply / is it really at 5V?

I went back to the post with a like to the transducer, and it shows 5V for the Vref. This should come from a 5V regulated sensor supply on the ECU. It should also use a conditioned sensor ground if possible (I don't know if the MS ECU has specific sensor grounds).

MS does have a conditioned sensor 5V and sensor ground. That's where the transducer is connected. Checking it for 5V is a good idea, i will add that to the list.

Mustang5L5 · Jun 2, 2026

Actually...for all you wide-band gurus out there. Does an excessively rich condition (<9AFR) sometimes read lean artificially on a wideband?

FastDriver · Jun 2, 2026

Absolutely can. In fact, if you had a leak allowing O2 in upstream of the WBO2, the computer would read that as lean no matter how much fuel it dumped trying to compensate. Or, if you were so rich it misfired, oxygen gets through without burning and that makes it look lean.

I'm stimied. Really thought it'd be one or the other. Have you ever datalogged voltage when revving? Just curious.

Another thing bugging me is the fact that you're getting an increase in pressure when the car is running. How can that be? The regulator doesn't care about voltage. More fuel is being used. A restriction on the backside doesn't just show up suddenly because the engine is running, and if it did, then what causes the drop?

I don't remember doing a rev test with my vacuum disconnected. So, just wondered what I'd see. Then I plugged it back in and did it again while filming the gauge:

Shared album from Chris

Tap to view!

photos.app.goo.gl

Never used google to host. Lemme know if there's any issue or barrier to watch, please. I wish I had also tested the KOEO pressure. I'll take a look next time I remember and am with the car.

Edit:
Is the sound jacked up? First time merging vids with google photos & hosting here. but basically it stays steady as a rock with vacuum unplugged. Then, in the 2nd part with vacuum plugged back in, it idles at 32-34 and jumps right to 40ish with quick tip-in. 2nd time, it bobbles slightly lower after letting off and quickly recovers back to 32 or so.

Mustang5L5 · Jun 3, 2026

FastDriver said:
Another thing bugging me is the fact that you're getting an increase in pressure when the car is running. How can that be? The regulator doesn't care about voltage. More fuel is being used. A restriction on the backside doesn't just show up suddenly because the engine is running, and if it did, then what causes the drop?

That's what's confusing me. The reg has a spring. It's not going to just loosen up because the engine is running. It still requires the same force to push that spring so pressure should be the same at 12v or 14v I would think. The mech gauge only went up a few psi with the engine running. I think i buy that.

I do see a bit more consistency with this reg than the Kirban. I see a few things here that make me want to test out a few more things. It's certainly possible the reg was contributing to the problem, but i wonder if a few other things are compounding this

Incorrect injector settings
incorrect battery voltage calibration

I *think* batt calibration in Tuner Studio will affect how sensors read. Essentially you program in an offset to match the voltage the MS sees vs what actual batt voltage is. My TS dash does show the same voltage as battery, but I may have used the wrong calibration to get there. I also need to test my 5V sensor power/ground to see what those actally are.

I can see your vid. That's what i'd expect to see. I'm going to try a simulated rev test by idling the engine, hooking up the mityvac, pull a little vac and release. I expect the pressure to shoot right up again. I'm also going to turn accel enrich off in the tune and see what I datalog. This might be a tune/calibration issue all along and I was chasing ghosts.

That's a hell of a lot easier to deal with than dropping tanks and such. I'm kinda pissed i tore my entire fuel system apart for this. Problem is the car runs/drives pretty damn good. It just bogs when I jump on the throttle. I saw the lean spike, and dip in FP, but now that ithink about it it's possible I was going pig rich.

hoopty5.0 · Jun 3, 2026

I guess I don't understand. My MS (is admittedly ancient) but all it does is turn the fuel pump on and control the injectors.
The pump moves fuel and the regulator creates the pressure.
If the key is on, engine off - injectors are doing nothing, yet FP still fluctuates? (injectors swapped, no change there, right?)
I just cant help but believe the pump is the issue. Either it has a voltage supply issue, or has a fuel supply issue (fuel sock, cavitation, etc.)

I cant wrap my head around the issue being in the software. I can see small fluctuations happening but not the large sweeps.

Can you hardwire the pump to a battery so it runs separate from the relay and see what happens? I dont think its the regulator now that you have changed it.

Mustang5L5 · Jun 3, 2026

This one is just turning the pump on too. It's rather dumb in the fact it's only on/off. The only adjustments MS can make is how long it opens the injector for.

Pump is essentially hardwired to the battery now. I have a relay in line but I can jump right across the relay or just command it straight on. DOing the same thing essentially.

Key on, engine off, the system is doing exactly what it should be doing. Applying vac to the reg, i can make pressure go up and down sharp and crisp. That's the graph in post #411.

Mustang5L5 · Jun 3, 2026

Heres some more data.

Fired up engine. VAc port on reg connected to my mity vac. Just letting the engine idle I applied and released vacuum. The response was as expected. Release vac and pressure goes up. No dip down at all. No change in AFR. Aside from looking at tuner studio you couldn't really tell i was forcing an 8-9PSI swing in pressure. Again..prob what you'd expect to see. Based on this test and the test in #411...all is well.

SO now i go into the tune and turn off Accel enrich. Not fully but I put the numbers waaaaaay down.

Then i give it a nice rev to 1500RPM or so. Nothing high demand. Fuel pressure is locked statically to 39psi. It shouldn't change.

But it does. I lose 6-8psi or so during this blip. What is interesting is the AFR is at 12.0 (messing with pressure and VE is screwing up my AFR) and during the blip it drops to 11.8 and then instant 19.0. (I think this is actually a excessive rich condition) After this spike it immediately drops to 13.0. Pulse width goes from 2ms at idle, to 9ms during this spike, but my total fuel correction is only 104%. Of course, now it's entirely possible my entire VE table is screwed up and those cells are just fuel heavy. That particular cell is in a low demand area of the table but seems high.

I think i may have gotten stuck in a loop. I (and the software) read that cell as lean (when it was really rich) and kept adding more and more fuel. I then got hung up on accel enrich tuning by continuing to rev in a no-load cell, seeing more lean spike (when really rich) and keep adding more fuel.

I almost wonder if i should make a brand new tune from scratch based on base tune...just to see what happens with a more conservative VE table and turn accel enrich down.

Could also be the fuel sock. But writing a custom fueled-down tune just to see what happens is easier.

hoopty5.0 · Jun 3, 2026

Have you talked to Steve at all? I have his number if you wanna run anything past him.

Mustang5L5 · Jun 3, 2026

hoopty5.0 said:
Have you talked to Steve at all? I have his number if you wanna run anything past him.

I think i've had a slight epihany on what's going on here.

I think i got my injector dead times completely wrong. The numbers I used would definitelty hang the injectors open way too long.

Mustang5L5’s progress thread - Rear window fun

mechanicus terribilis

I was uncomfortably high & wearing a helmet

That is…until I whipped out my Bissell

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

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

mechanicus terribilis

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

mechanicus terribilis

That is…until I whipped out my Bissell

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