Fuel Both banks lean, fuel pressure is at 30

Xenogenesis

New Member
Dec 17, 2019
10
0
1
18
Tennessee
Okay, so I've got a 95 Mustang, 3.8 V6. Been tinkering on it for a while and haven't quite found the issue, but recently found the fuel pressure is reading 30 at idle, and I believe it's supposed to be 38-40 or so at idle. It reads both banks lean, and I believe this could be why. Key on, engine off, it reads about 35 and doesn't drop.

I replaced the fuel pump, fuel filter, gas tank, all 6 injectors, and the fuel pressure regulator. I imagine the issue is pretty obvious, something maybe the fuel pump is going out again? I replaced it quite a few months ago, but I don't think it should be out already. Maybe I should go performance?
 
  • Sponsors(?)


wmburns

SN Certified Technician
Aug 14, 2009
5,606
464
164
Houston Texas
Explain HOW you measured the fuel pressure.

Why is this important? Because the fuel pressure is intake vacuum referenced. At idle the intake is under a VACUUM. This means that any measurement using an external gauge will read the difference between outside atmospheric air and the fuel pressure. So that means to get the actual intake reference fuel pressure it will be necessary to ADD the intake vacuum to the external gauge reading.

Soooooo 30-32 PSI at idle is a reasonable atmospheric gauge fuel pressure. But if you want to double check, then disconnect and plug the fuel pressure regulator intake vacuum reference line. The fuel pressure as measured by an external pressure gauge should immediately jump to around 40 PSI.

What have you done to rule out a vacuum leak? How about the EVAP system? How about in the EGR system?

How have you tested the MAF sensor for accuracy?
 
Last edited:

Xenogenesis

New Member
Dec 17, 2019
10
0
1
18
Tennessee
Explain HOW you measured the fuel pressure.

Why is this important? Because the fuel pressure is intake vacuum referenced. At idle the intake is under a VACUUM. This means that any measurement using an external gauge will read the difference between outside air and the fuel pressure. So that means to get the actual intake reference fuel pressure it will be necessary to ADD the intake vacuum to the external gauge reading.

Soooooo 30-32 PSI at idle is a reasonable fuel pressure. But if you want to double check, then disconnect and plug the fuel pressure regulator intake vacuum reference line. The fuel pressure as measured by an external pressure gauge should immediately jump to around 40 PSI.

What have you done to rule out a vacuum leak? How about the EVAP system? How about in the EGR system?

How have you tested the MAF sensor for accuracy?
I used a fuel pressure tester kit that hooks up to the injector rail bleeder valve.

Now that you mention it, I looked up the EVAP, and that's the little thing with wires, connecting two vacuum hoses, from the intake/valve cover to inside the fender somewhere on the passenger side, correct? Because the line after the EVAP is so rotten it's not funny. I'll definitely replace that, and the vacuum hose. Where does it run to?
 

wmburns

SN Certified Technician
Aug 14, 2009
5,606
464
164
Houston Texas
and the vacuum hose. Where does it run to?
The fuel pressure intake vacuum reference line goes from the intake to the fuel pressure regulator.

IF the EVAP vapor line is rotted out between the manifold and the EVAP purge solenoid I could easily see this as a vacuum leak source. In the same way as a failed open EVAP purge solenoid could also cause a vacuum leak.

For the 1998 and older Mustang, the EVAP vapor line goes from the intake vacuum port to:
  • the Purge flow sensor
  • The Purge flow solenoid
  • The charcoal canister (or vapor value) in the fender well.
 
Last edited:

Xenogenesis

New Member
Dec 17, 2019
10
0
1
18
Tennessee
The fuel pressure intake vacuum reference line goes from the intake to the fuel pressure regulator.

IF the EVAP vapor line is rotted out between the manifold and the EVAP purge solenoid I could easily see this as a vacuum leak source. In the same way as a failed open EVAP purge solenoid could also cause a vacuum leak.

For the 1998 and older Mustang, the EVAP vapor line goes from the intake vacuum port to:
  • the Purge flow sensor
  • The Purge flow solenoid
  • The charcoal canister (or vapor value) in the fender well.
Thank you.

I have another question for you, I've looked all over trying to Google it, but I don't see anything specifically like it?

My engine seems to startup idle just fine for a little bit, until suddenly it just starts misfiring quite a bit. Like something drags on it and it starts misfiring, even the RPMs go down. I've found threads about misfiring, but mine seems to be fine until suddenly it'll drop and misfire.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,606
464
164
Houston Texas
This could be:
  • A charging system problem. Low voltage. excessive AC ripple, bad alternator, bad battery, loose cable.
  • intake vacuum leak.
  • ignition problem (moisture in the spark plug wells for example).
  • excessive EGR flow.
  • Unstable fuel pressure
  • Unstable CKP sensor.
Start with the alternator and charging system. Don't cut corners here! Unless the charging system is in tip top shape a wild goose chase could be in your future.

Howto perform charging system voltage drop test
 
Last edited: