Low fuel pressure downstream of fuel rail, good pressure upstream

[FYWW]

Pretty sure I know the issue here, but wanted to see if anyone else has had the same experience:

Backstory: I have a 96 GT with 106k bought from a private seller about a year ago. 28 years is a lot of time for shenanigans, so I won’t go into what has been done to this car. All in all it’s been very well maintained, though there are many original parts that are on life support.

Judging from the carbon build-up on the plugs and O2 sensors, The vehicle runs very rich. I’m getting codes on all four O2 sensors (P0135, P0141, P0155, P0161). I did all the diagnosis of the heater elements and electrical back to the PCM. All of those things check out.

Issue at hand: Low fuel pressure.

What I’ve done:

• Checked the pressure at the schrader valve. On prime, I was getting a whopping 4psi. No, not 40. 4! When I started up the vehicle the needle slowly climbed up to 30psi.
• I then removed the vacuum line from the pressure regulator and the psi rose to 40psi. Replaced the vacuum line and it went back to 30. This is what it’s supposed to, so I assumed the regulator was doing its job. Right? RIGHT?
• I wasn’t able to pinch off the return line as it’s nylon or some other hard substance.
• This lead me to believe it was a bad fuel pump. I dropped the tank and pulled out the pump, and I’m glad I did. It looked nasty, and possibly the original Ford unit.
• I replaced it with a Delphi unit as well as a new Motorcraft filter.
• I also removed all of my fuel injectors, cleaned them up, and replaced their o-rings. All injectors ohm tested between 14.9-15.1 ohms, and had clean spray patterns.

I was excited to get this all buttoned up and see if my fuel problems were a thing of the past. Spoiler alert: They weren’t. Still had the same exact issue, which was disheartening.

Next I did something I should have done before loading up the parts cannon, and that was test the fuel pressure upstream of the fuel rail. Primed the fuel pump and my pressure gauge immediately jumped to 30psi. Primed a 2nd time and it jumped to 38 or so. Started the vehicle and pressure held steady.

So … Bad fuel pressure regulator? Some other restriction within the fuel rail system? I inspected the regulator and found that 1. The o-rings and gasket were trashed, and 2. The damn thing is bent! Looks like someone didn’t seat it flush before cranking down the bolts with an impact driver, causing it to bend. See photo. Thankfully it didn’t bend the mating surface on the fuel rail. Surprisingly there isn't any fuel leaking from this gap, nor is there any fuel leaking from the vacuum stem.

I’m going to replace the regulator anyway, but wanted to share this in case anyone else has had the same/similar issues and if this fixed it.

Thanks!

 

[Mcmahst]

What is the vacuum at idle and at 2k? These cars have a notorious issue with running rich due to a vacuum leak. It’s got so many vacuum hoses on it, it’s not surprising that is an issue. Check all that out before you start to replace more parts.

 

[FYWW]

What is the vacuum at idle and at 2k? These cars have a notorious issue with running rich due to a vacuum leak. It’s got so many vacuum hoses on it, it’s not surprising that is an issue. Check all that out before you start to replace more parts.

Tons of vacuum lines! While waiting for my regulator to arrive, I took the time to remove my fuel rail and check for any blockage as well as inspect all the vac lines for any damage. All clear. My intake plenum had a good amount of carbon residue, so I got that all cleaned out. Might have to Seafoam this later, or hopefully it'll all just burn out on its own.

I installed the new fuel pressure regulator and it has resolved the original issue. After priming the pump a few times, my pressure gauge got up to 33psi. Started up the car and she stayed right around 33 or so. I let it warm up for a bit and then pulled off the vacuum line to the regulator. Pressure rose to 43, and then back to low 30's when replacing it. I did this a few times to check for consistency. Checked all vacuum lines for any signs of leakage and found none.

I then slowly opened up the throttle plate with the vacuum line on and the pressure remained steady at about 33 or so. Then I did a few quick revs up to 3k and the needle jumped up and back down. I believe this is expected behavior.

NEW issue is that my idle in Park or Neutral is a bit high at right around 1200-1300. I suspect I upset it after removing and cleaning my plenum (also had the battery disconnected most of the morning). When I rev it, it takes its time coming back down. As soon as I throw it into gear, it drops down to around 800-850. I unplugged the IAC connector and the idle dropped to normal range. Plugged it in, and it went back up.

I've had this same issue with another vehicle once, so now I just have to dig back into my brain vault and figure it out. Possible that I just need to drive it a bit and let the computer relearn? If anyone has any advice on this for 4.6 specific Mustangs, I'm all ears.

 

[Bullitt347]

There should not be much of any fuel pressure downstream of (after) the regulator.
That is just a way to dump fuel back into the fuel tank. No real pressure required.
Not sure why this is mentioned in the thread title, as it is not mentioned in the text describing your issue.

 

[FYWW]

There should not be much of any fuel pressure downstream of (after) the regulator.
That is just a way to dump fuel back into the fuel tank. No real pressure required.
Not sure why this is mentioned in the thread title, as it is not mentioned in the text describing your issue.

My error, it should have read downstream of fuel rail (corrected title). I tapped into the main fuel line before it connects to the rail and tested pressure there. In any case, the initial issue has been resolved with a new regulator. And you are correct, the line after the regulator is just a return. Sorry for the confusion.