Mustang5L5’s progress thread - Boom

[Mustang5L5]

Just happened to be browsing through the data log and noticed something interesting.

WOT pull. I hit 5100RPM before upshifting and letting off. Fuel pressure is interesting.

As i rolled into the throttle and got under load, pressure dropped to 29psi. Recovered back to 35-37psi when I let off. Duty cycle was 45% but my AFR was still lean at 14.2 for that KPA range.

190LPH pump. I would think that's enough. No? I definitely need to go back and recheck the static FP setting to make sure it's at 39psi vac off.

verticle blue line is right at 5100 RPM. Now i have to go back and check other logs to see if i'm not giving enough fuel at WOT.

 

[Noobz347]

190LPH pump. I would think that's enough? No?

Is 'maybe' a good answer?

When you start producing enough HP that stock fuel rails and lines begin to resist that fuel volume then a high pressure pump is recommended.

They [resist] the pushback, so to speak.

I do not recall your entire setup.


Also, see if you can get a read on pump voltage when this occurs. It's possible there is a voltage dip at this rpm that you're not aware of. This could effect spark too. I saw a scenario similar to this on a dyno-run. There was an unexplained dip in the power curve that started the investigation.

 

[Mustang5L5]

302 with TFS heads, cam and ported cobra. Maybe 325 crank HP. Should be in the realm of what a 190 pump should support.

I need to dig into this. Pressure shouldn’t be dropping off that much at WOT. I need to recheck my vac off pressure and maybe change out the fuel filter and shake it out. It should be new.

 

[Mustang5L5]

looked through more datalogs. Looks like just revving the engine causes FP to drop rather than go up. I see it rather consistently. I think something is going on here. Will need to investigate further.

Sits at 32PSI with engine running. ABout where you would expect. When you rev, it drops to 29psi and then recovers. I would expect it to rise to 39psi.

First thing i'm going to do is test the sensor and verify the MS setup is calibrated right. I can buy a cheapo on Amazon, plumb it into a pipe nipple with two coupling and fill it up via compressor and gauge to various pressures just to make sure the MS is reading properly.

 

[AeroCoupe]

I know you know this but I am going to "talk it out" and make sure I have a good understanding of how this works so please correct me if I am wrong. I think we all understand that the fuel pressure should increase as the RPM's go up due to the vacuum decreasing. It does this to maintain the same pressure at the injector when the vacuum is high or low. The higher the vacuum at idle the less fuel pressure you need the pump to supply to get 39 psig at the injector due to the engine literally sucking fuel from the injector when it opens. At WOT the vacuum reference is going towards zero so you will see the pressure increase to the pressure that the regulator was set at with the engine idling and the vacuum line disconnected and plugged.

Falling fuel pressure with increasing RPM indicates a pump issue (mechanical or electrical) , filter issue (fuel pump sock or downstream filter), kinked fuel line, or a possibly an issue with the regulator.

This is all predicated on your fuel pressure readings being accurate so as usual you are on top of that. Interesting that you mentioned the AFR was indicating it was lean so that to me means the fuel pressure reading could be accurate. Will definitely be curious to see what you find.

 

[Mustang5L5]

I think you got it. I do think these readings are accurate. Looking at my logs, i see this as a trend which makes sense as when I was trying to work on Accel enrichment, i was having to add a lot more fuel than I expected and still seeing a lean spike.

I have some parts on the way to try and verify the reading is correct (i think it is) but I think I'd drop the fuel filter and take a look at that next. I have a full tank of gas so not really looking to drop a tank right now.

 

[AeroCoupe]

Yeah, that is where I would go as its cheap and easy to do. As recently as you reworked your gas tank I would think you are okay there. Do you have a stock regulator or aftermarket?

 

[Noobz347]

Falling fuel pressure with increasing RPM indicates a pump issue (mechanical or electrical) , filter issue (fuel pump sock or downstream filter), kinked fuel line, or a possibly an issue with the regulator.

It indicates a decrease in fuel [volume] and I agree that these things you listed are possible causes.

On many Fox rigs (including mine), the electrical system just wasn't up to the task. You guys that put even more of a demand on that system than I do, always leaving me suspecting wiring and power supply.

I've regulated my coil and fuel pump. For me, this made things stable on the dyno.

Question: If running electric fans, are they disabled by a WOT circuit?

 

[Mustang5L5]

Do you have a stock regulator or aftermarket?

It's a Kirban regulator. It's 20+ years old. I know they can be rebuilt, but not sure where I can find a rebuild kit

Question: If running electric fans, are they disabled by a WOT circuit?

The ECU disables the PWM controller. But i'm seeing the same thing at idle with the fans off if I rev the engine. Voltage holds at 14.4+ at idle.

Pump is a Walbro 190 I installed a few years ago. I have a borescope I can use to peek in the tank, but right now it's full. I can siphon some off for the snowblower this winter and at least make sure it still looks clean in there. Pulling fuel filter and cutting that open might give me some clues too but that filter has less than 500 miles on it. Pump too.

I'm leaning FPR right now, but I need to troubleshoot a bit first. I do think this is real now as i go back and look at datalogs of various situations. Any rev or throttle blip or rolling on the gas results in a drop to 29psi. FPR holds around 32-34 during normal cruise/idle. It should spike to 39.

 

[AeroCoupe]

REPLACEMENT DIAPHRAGM FOR KIRBAN STYLE ADJUSTABLE FUEL PRESSURE REGULATOR

www.kirbanperformance.com

 

[Mustang5L5]

Pricey considering I can get the whole thing new for $150. There are a few O-rings internally as well. Those are cheap but i'll need to spend the time to measure and buy a few to try out. Not like i don't have time.

1987-1990 1991 1992 1993 Ford Mustang GT 4.6 5.0 Shelby Cobra BILLET ALUMINUM ADJUSTABLE FUEL PRESSURE REGULATOR

www.kirbanperformance.com

I'm leaning towards thing being the issue considering it's 20 years old and did sit for 8 of those years with old gas inside while the car sat stored. I prob shoud have rebuilt it before or at least opened it up.

But i'm not there yet. Let me check a few things first. Mainly I want to check the pressure sensor setup on MS first and make sure 29psi is 29psi and make sure something funky isn't going on there.

 

[Mustang5L5]

Heres a log snapshot. Idling at about 32psi. Blip throttle in neutral. Injector pulse width goes up. Fuel pressure drops to 29 and AFR goes up.

It;s as if the FPR does not respond to the vac dropping. The fuel pressure drop of 2-3psi could be due to the fuel injectors squirting a lot more fuel for that time.

 

[Noobz347]

Your 'blip test' pretty much eliminates the possibility of being the regulator.

If fuel pressure dips exactly when throttle demand rises, that happens before the regulator.
A regulator cannot cause a drop when manifold vacuum goes away; It would rise instead.

 

[Bullitt347]

You might be chasing your tail in this scenario.
I can't get an accurate reading on the amount of time involved during this blip of the throttle, but given how many things have to happen to stabilize fuel pressure from a change in demand, it may not be something that can be "fixed".
Vacuum drops, this has to act on the fuel pressure regulator diaphragm. The fuel used in the rail has to be replaced to get rail pressure back to what it was.
If all of this takes place in fraction of a second, it may be physically impossible to maintain the exact same fuel pressure, or gain pressure in that time frame.

If you could use your cursor on the data-log to measure start/stop time frame of the dip in fuel pressure it might give some clarity with what you are dealing with.

 

[Mustang5L5]

Lets ignore the blip. You are right it's too quick.

Originally what got my attention on this was a 2nd gear WOT pull to 5100RPM in which I checked fuel pressure and saw it was dropping to 29psi.

I guess the question here is wouldn't one assume the FP to rise to 39psi (FP with vac off) during this sustained pull provided the fuel pump can sufficiently deliver enough fuel?

Snip below. I don't have RPM plotted but blue vertical line is right at 5100RPM. You can see MAP (red line) rising to 98 kpa and fuel pressure (bold white) falling to 29psi as RPM climb.

Maybe I am chasing my tail here and nothings wrong, but I'm going off the assumption that under this sustained pull FP should rise to 39?? Or is that wrong?

 

[Bullitt347]

Your MAP reading at 5,100 rpm is 98kpa.
This is at WOT?
How is that possible?
100 kpa is 14.5 psi which equals atmospheric pressure, which would be a key on engine off reading from the MAP


What is MAPdot? In Green.

 

[Mustang5L5]

MAPdot is rate of change in MAP. So a reading of 7 indicates MAP is still climbing.

WOT with throttle blade fully open should equal 100kpa or close to it. If throttle blade is partially of fully closed you’ll see less kpa

For example at 2:45


View: https://youtu.be/vAJGcgKxgsU?si=ZGImXQiPybHiNvbe

 

[Kid wita 5oh]

WOT with throttle blade fully open should equal 100kpa or close to it. If throttle blade is partially of fully closed you’ll see less kpa

Unless you have a restriction on the engine with Intake being too small for heads and displacement (unlikely in your situation)

 

[AeroCoupe]

I believe the intakes were ported by TMoss and the rest of the combo is well sorted out (Ed Curtis custom cam etc.).

 

[Mustang5L5]

I'm not seeing signs of restriction. kpa numbers are where I would expect them to be.

I think I have my list of things to check. Just need time. Car is kinda put "away" so this will take place when i get some time. I might need to siphon some gas out of the tank to inspect a few things with a borescope. No mood to do that right now.