Mustang5L5’s progress thread - fuel pressure shennanigans SOLVED

[Mustang5L5]

I think this confirms that the MS software is using MAP as a FP reference and giving you the dP across the injector. From what I can see in your picture, the mechanical gauge is reading 37 psi (confirmed in your text) and the ECU is reading MAP at -7.9 psi. If the transducer's raw voltage reading (please check the raw voltage and do the slope offset by hand to confirm) is correct for the gauge pressure, then the math is: transducer pressure (37 psi) - MAP (-7.9 psi) = 44.9 psi delta pressure. This is roughly the 45 psi shown on the MS software screen for fuel pressure.

I was running the FP fixed. Vac line was removed and capped. SO manifold vac shouldn't be affecting FP directly.

But i graphed FP to MAP, and do see a relationship. They are almost inverse to each other. But this could be correlation and not necessarily causation. I added in PW (yellow) and the engine is trying to add fuel when MAP increased, corresponding to increased load on the engine.

As a second step, can you bring in the FP transducer as just a raw transducer input and NOT map it to fuel pressure? If the voltage or calculated pressure looks correct in MS as a generic element, not linked to any exact sensor feature, this would confirm that the MS software has more math behind the fuel pressure calculation.

I just figured that out in the software. SO now i can display it as raw volts and convert myself to FP. That should eliminate any sort of calculation done in the background.

But it checks out engine off, pump on pressure. 37psi on transducer = 2.0 volts = 37psi on my mech gauge. I'll have to start engine up tomorrow and see what it displays as raw volts when it's running. If it truly is 45psi then I should see 2.3 volts

Regarding the restriction, have you considered running an external line from the tank to the rail? This would be very dangerous if not done correctly and carefully, but might answer some questions. If you already have a bucket setup, you could also run the pump hat from a bucket with short lines to the rail and see what happens.

I'm still doing this with the tank on the ground, but buttoned up. Tryng to keep fumes to a minimum. Garage doors open and fans running, so not really ready to get crazy with misc lines yet.

But i am on the fence on slowly converting over to 6AN connections and braided lines up to the engine. I want to stick a gauge at the pump first.


But i do agree with you. There is a restriction somewhere, and something funky is going on with how my transducer reports pressure when the engine runs.

 

[Mustang5L5]

Well I’ll be….

 

[Mustang5L5]

No way could I go to bed without trying a few things.

SO went into the tune, deleted the FP "guage" i was using and instead created a generic pressure sensor that I labeled Fuel, but didn't define as fuel pressure. Gave it the correct values.

Loaded tune, key on/engine off and BAM 37psi like the mech gauge.

If it wasn't 11:12PM i'd fire the car up. I need to know. Thanks @WhiteCobra95 for sending me down this path. I know you mentioned it before but I took a different look at it tonight and went down a different google rabbit hole and found that post. It wasn't a configurable setting (which is what i was looking for before) but, instead, it was done as a background calculation.

DO i think this solves my restriction issue? I don't think so. But this solves my lack of trust in the pressure readings.

SO hopefully i can fire up car tomorrow, confirm the transducer is reading correctly and ditch all my misc fittings/gauges. 1 prob solved...hopefully.

 

[WhiteCobra95]

Any time! @nicholase deserves the assist on this one for catching it. I just didn't let up because everything else regarding your transducer setup looked fine (the specs, the wiring, your initial external tests, ...). The only question mark was the MS software. A lot of times these aftermarket EFI systems are very vague with how they're using a sensor, and unless you have a functionality flow diagram or the C-code, it's difficult to know what they're really doing with an input and how it's used in their controls calculations. Your mechanical gauge install and pictures helped to separate the issues and prove that we had two problems going on at the same time, which seriously compounded the difficulty in sorting things out. I'm really glad that one of the two has been identified and resolved!

Now it's on to what appears to be a mechanical flow issue. I like the idea of putting either the transducer or the gauge as close to the pump outlet as possible. I have a bad feeling that your hard line either has an obstruction or a kink somewhere. If the pressure at the pump outlet location looks good, then I suggest moving the gauge to 1) the output of fuel filter, 2) then the inlet of the hard line, and 3) then move it to the output of the hard line before the soft line to the fuel rail as the next nodes to verify. I suspect the fuel pressure reading will be higher than the set point when you measure upstream of the suspected flow restriction. The best scenario is that it all checks out to the hard line's outlet, then the suspect section becomes the soft line, fittings, and fuel rail, which would all be much easier to inspect, clean out, or replace.

Good luck and be safe! You're going to be smelling nothing but raw gasoline for some time!

 

[Mustang5L5]

Ah you are right. Credit to @nicholase as well.

I followed what he had said but didn’t see any settings in MS to adjust for the pressure differential. Didn’t even think that displaying the fuel pressure would run a background calc.

So now it’s displayed as a generic sensor so hopefully when I fire up the car today it shows the same as my mechanical now, and I can finally adjust FP to 39-40psi and trust that reading.

As for the restriction issue….lets see what this datalog looks like. I’m very curious now considering the WOT issue seems to be an artifact of how MS displays a calculated FP. Plus, it appears I may have been as much as 9psi low this entire time I was tuning. That certainly didn’t help

I’ll get to it a little later today Don’t really want to fire the car up at 9am on a sun.

 

[AeroCoupe]

I’m definitely getting a remote tune when I install the MS in my Coupe. You guys are so far over my head on this stuff. Learning a lot reading along but no way am I comfortable with it. Glad you have hopefully figured out the pressure discrepancy.

Can you lay out your fuel system from hanger to to the fuel rails and back to the tank? I know it’s not stock but I’m unsure of what it is. Hope that makes sense.

 

[Mustang5L5]

I think we got it. Not 100% positive yet until i go for a drive, but I think this was a few errors compounded which messed up the data I was reading.

#1 was the injector data. My settings made them way too rich, so I saw signs of this and it's what got me thinking i was somehow dumping way too much fuel.

#2 was the transducer being logged as FP in MS causing it to really report differential pressure. I didnt know this is how it would act, and in my defense it wasn't really clear. It took me an hour of googling to find some obscure comment suggesting this.

#3 is when I fired up the car, i saw that the mech and transducer match! Victory! I also then saw my fuel pressure was actually set way low since I was setting it based on the transducer. 39psi on the transducer was actually 29 psi.... The magic 29 I was chasing for 6 months. Bumped the pressure to 40psi (transducer appears to read about 1psi high) started engine and it ran great. Gave it some blips and ROCK SOLID AT 40 PSI!!

WHoever said my static pressure was actually set to 29psi was 100% right. I just couldn't see it because I was trusting my pressure readings. After all, it read correct full range with the engine off.


Of course, I need to drive this thing now to actually see. My VE table is a mess due to apparently setting it to 29psi and driving around. Apparently that's not good.


SO yeah...6 months of tearing my fuel system apart over some MS blunders.

 

[nicholase]

Any time! @nicholase deserves the assist on this one for catching it.

Ah you are right. Credit to @nicholase as well.

So we get cookies right?

WHoever said my static pressure was actually set to 29psi was 100% right.

....Check to make sure you don't have a fuel pressure differential (PSID) enabled somewhere in your software. If you do, that is 100% your issue for everything.

It needs to be PSIG or PSI "guage" for the time being. I can elaborate more on that if this turns out to be the case.

TL;DR
Disable PSID and select PSIG. Start your car. Disconnect and plug your FPR vacuum line. Check your PSIG in your software. Betcha it says your base FP is 29 psi engine running vac line removed.

Prepare for a brain dump....

PSID or differential reads the difference between your rail and intake. It's getting your FP from the rail. It's getting your intake reading from your MAP (or other vac sensor in the intake, not familiar with your software/hardware exactly). However when thats enabled you don't actually see either of those values. You only see the calculated differential.

So, when you unplug your vac line from your regulator to set your FP the software is comparing rail FP against your vacuum in the intake. For easy numbers let's say you have 20" engine vacuum at idle. To do the differential calculation first we need to convert to like values for each side of the equation. It's easiest to convert vacuum inches of mercury to psi. The equation is divide vacuum by 2.03692 for the negative psi value. But 2 is close enough. Or in half. So 20" vacuum = negative 10 psi. The differential between positive 39 psi FP and negative 10 psi intake conditions is 49 psi.

So when you set your base to 39 psid on the software, you were actually setting it about 10 psi lower than you thought.


The following equations are for an engine vacuum at idle of 20"hg (or negative 10 psi intake conditions at idle) Your actual engine vacuum will differ of course. The equation works regardless, just enter your actual numbers.

-That would explain your KOEO FP of 29 becasue base FP is actually set at 29 psi. The differential calculation there would be base 29 fp + 0 intake vacuum = 29 psid. Exactly what you see KOEO.

-That would explain why it goes up to 39 in the software when you start it at idle with the vac line off the regulator as you mentioned. The differential calculation would be positive 29 base fp + half of engine vacuum (negative 10 psi) = 39 psid. Exactly what you are seeing.

-That would explain why it reverts to 29 psi at WOT becasue it's actually set to that. The differential calculation would be base positive 29 psi + 0 vacuum at WOT = 29 psid. Exactly what you're seeing.

-That would explain the lean spike becasue your FP is actually 10 psi low.

-That would explain why you were adding so much fuel and not seeing the outcome you expected.

It actually explains all of it. It's gotta be it.

Glad I could help steer the convo to differential and calibration. It was the only thing that could explain some of the behaviors. Just glad I didn't steer this in the wrong direction.

Now.... fix the tune . Its gotta be all outta shape lol.

Gonna feel good to get the gas smell outta the garage huh? The only thing worse than that is diff fluid and friction modifier that stuff latches on like a tick.

 

[FastDriver]

First, full credit to @nicholase for putting us onto the PSID/delta-pressure explanation. That was the key mechanism, and he nailed it.

But this is also why I kept harping on the old data and why I wanted the raw transducer voltage/generic sensor value. The transducer itself could be perfectly accurate and still mislead you if the MS “fuel pressure” channel was not raw rail pressure.

That’s what I was getting at here:

This post is the smoking gun. Your base pressure is and has been 29psi, not 39.

And here:

This is just telling you that base fuel pressure is 29. then when you run the car and pull the vacuum from the AFPR and get 39, that base is suddenly higher. How is that possible? I think the answer is that it isn't possible. Slightly more voltage, fuel being injected at idle, etc... these do not account for the 10psi change, but adding the intake vacuum in psi to the fuel pressure at the rail does.

And on Corral:

MAP and FP are basically mirrors.

And then

Put a multimeter directly on the transducer signal wire while the engine is running and compare voltage to what MS reports.

The raw transducer voltage would have allowed you to see the difference between calculated FP and raw sensor-derived FP. Because At that point, calculating pressure directly from raw voltage would have shown whether the MS calculated FP channel matched the transducer voltage alone.

The raw voltage/generic sensor test mattered because it separated three different things that were getting blended together: the transducer accuracy, the raw rail pressure, and the MS calculated fuel-pressure channel.

Ok... done with credit and focus on tge past...

Now that actual base pressure was 29 instead of 39, I think you probably need to rethink more than just the VE table. The idle AFR preference may be part of the same story too. If the car was really idling and driving around on 29 psi base, atomization/mixture quality was probably worse, so needing 13.6-13.8 to idle cleanly may have been compensating for that rather than telling you the engine inherently wants to idle that rich.

So yeah, the VE table is obviously out of shape, but I’d also revisit injector data, AE, idle AFR, and idle timing after setting true base pressure. A lot of the tune may have been built around a false fuel-pressure assumption.

 

[Mustang5L5]

Now.... fix the tune . Its gotta be all outta shape lol.

Yup. It's rich all over. Need to pull out a lot of fuel.

 

[HotFox]

Following... glad you are on your way!
You make me glad I run a carburetor on my car. Although I am currently building my new engine with a EFI intake with intentions to convert to EFI someday, it probably won't be MS; it will likely be a Holley system.

 

[WhiteCobra95]

Don't forget to hook up the vacuum port once you're ready to start tuning again!

Also, with the vacuum port plugged in you'll have to check your rail pressure measurement / setting by [rail pressure measurement PSIG] + [Ambient pressure - MAP] = 39 psi

And what was that about cookies???

 

[hoopty5.0]

Nice work all. Glad you got it sorted.

 

[Mustang5L5]

Don't forget to hook up the vacuum port once you're ready to start tuning again!

Also, with the vacuum port plugged in you'll have to check your rail pressure measurement / setting by [rail pressure measurement PSIG] + [Ambient pressure - MAP] = 39 psi

And what was that about cookies???

Yup. Had to grab some vac line for that. Should be taking care of that today. Once i get that crossed off the list I pretty much have to redo this entire tune starting with idle and my VE table.

 

[General karthief]

Tech like this is kind of a foreign language to me but still I'm interested somehow.
This reminds of what someone said about EFI when it was first introduced, computer controlled EFI makes a carburetor look like pouring gas on a rock and smack'n it with a hammer.

 

[nickyb]

I was right all along, "loose nut behind the keyboard"
Glad this nightmare is over. Congrats.