Mustang5L5’s progress thread - fuel pressure shennanigans SOLVED

[Noobz347]

Two things come to mind:

First, physically verify the feed and return paths from the tank to the rail and back again. Don’t assume they’re correct because the connectors fit or because fuel flows. Confirm pump outlet → feed hard line → rail inlet, and regulator outlet → return hard line → tank. Make sure the two paths were never swapped, even if you’re sure they weren’t.

Second, completely bypass the car’s return line for a test. Disconnect the return line after the regulator and route the regulator outlet into an empty gas can with fuel-safe hose. Then run the pump-prime test again.

This is not a “does fuel come out?” test. This removes the body return line and tank return path from the equation. If pressure changes, the problem is downstream of the regulator. If pressure does not change, we can stop chasing the body return line.

 

[Mustang5L5]

One thing to add. I only added the fuel pressure transducer after I put the 47# injectors on.

Has anyone ever heard of an injector bleeding off pressure over a certain point? In this case anything over 29psi discharges? I have noticed the cold start takes a lot more cranking now to fire off. Could be tune but i didn't change anything with regards to my cranking. It fired off nice before. Makes me wonder...

I want to assume the injectors are good, but at this point i'm willing to take wild shots. I wonder if i should pull plugs and listen/smell for gas spraying in while I key prime.

I'm at that point where I'll entertain wild ideas so fire away.

 

[Noobz347]

190 l/hr Walbro pumps in particular, have (or had) a 60 psi internal pop-off plug.

Because of this, anybody and everybody that ran an FMU went straight for the 255L pumps.

From, "Fuel Flow Forensics" in the [Resource] section:

 

[FastDriver]

Afraid to say it again, but I don't see where you directly addressed the fuel filter orientation. It should have a flow arrow or "this side to engine."

 

[Kid wita 5oh]

Dead head the return, install gauge, briefly key on should hit @100psi...
That will rule out the supply side

 

[Noobz347]

If that happens and it's one of the 190L pumps I mentioned above, you're done.

That pop-off doesn't reset.

 

[Mustang5L5]

I pulled plugs tonight and stuck a tube in the cylinder to listen/smell for gas. Nothing. No sign of any leakage from what I can tell. Almost tempted to pull the rail and install my 30s and see if anything changes.

That’s what’s frustrates me. All the changes I’ve made and everything is the same. Not even a 1 psi difference.

I did voltage check and it’s minimal drop now. Even that puzzles me. I had a lot of drop before I rewired the pump. Now it’s essentially gone and same readings.

I stuck a mechanical gauge on the rail and verified my readings. As far as I can tell it’s real.

Volume test checks out. At 12v I pumped 1 gallon (give or take) at 60 seconds. You can do the math there.

Going to order up some fittings and braided line to put a mechanical gauge on the cowl temporarily.

Here’s what puzzles me. Key on prime will not go past 29psi. Even doing 2-3 in a row. Engine starts and it will go to 39psi with vac line off. But engine off and even just running the pump in test mode and it’s stuck at 29psi. That’s also the same psi that the pressure drops to when I go WOT

Again, no voltage drop on the pump. Also, same behavior with two different pumps.

What am I missing?

Going to recheck a few other things here, but I’m 99% sure fuel filter flow direction is right too. I replaced the filter, and it’s the stock hanger so it only installs one way really with the flanges end towards the pass side.

I’m stumped on this. I’m at the point where the parts cannon is ready to fire. Not a single change has resulted in any difference in what I am seeing.

 

[Kid wita 5oh]

I replaced the filter, and it’s the stock hanger so it only installs one way really with the flanges end towards the pass side.

Nope think it's the other way...

 

[Mustang5L5]

I’ll double check tomorrow. At this point everything is worth doing again.

 

[Rdub6]

I can’t add much to solving this dilemma, but from my old pics, I can confirm the larger flanged end of the filter faces the driver side.

 

[FastDriver]

Ooh... Please let me get one. It's been a long time since I got one of these "guess my problem" mysteries.

 

[Mustang5L5]

I may have misremember the orientation. I’ll get under there tomorrow and verify

 

[AeroCoupe]

Man you have checked everything I can think of and replaced pieces that would directly cause the issue. Fuel filter sounds like a reasonable thing to cause it.

 

[General karthief]

Can you monitor the vacuum at the regulator line to see what it does?

 

[Mustang5L5]

Man you have checked everything I can think of and replaced pieces that would directly cause the issue. Fuel filter sounds like a reasonable thing to cause it.

Stuck my arm under with my phone and a light and snapped a bunch of pics and from what I can see, the fuel filter is oriented correctly with the lines hooked up like shown in the above pic in post 350 and flanged end facing drivers side. So i made a mistake. I'll jack car up later and go look but from what I can see the fuel filter is installed correctly.

I also did replace it early on in this project and cut the old one open. Nothing was inside.

Can you monitor the vacuum at the regulator line to see what it does?

Going to get a hand vac pump and play with vac next. At this point i'm trying everything.

 

[nickyb]

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.

 

[hoopty5.0]

any chance that the return line is aerating the fuel around the pump and cavitating?

 

[Mustang5L5]

any chance that the return line is aerating the fuel around the pump and cavitating?

I plan to disconnect the return at the engine and flow into a bucket. That should determine if that is the case when I do that.


I'm also going to swap the injectors just to rule them out as leaking. It will take me less than an hour and one more thing to check off as done. I just have a stinking suspicion and want to see...

 

[Mustang5L5]

Confirmed fuel filter direction is fine.

Gonna see if I can swap injectors real quick to my 30#ers

 

[AeroCoupe]