Fuel pressure testing?

DAA

Premium Member
Supporting Member
2006 Jeep 4.0. All the FSM has to say about fuel pressure is:

Capture.JPG

58 psi +/- 2 psi. Doesn't say under what conditions though. Static, key on not running? Running at idle? Running under load?

Anyone care to comment on which condition should show 58 psi?

First key on, not running:


IMG_0390.jpg


Well that's pretty low. Didn't move after a few minutes.

So running at idle, though, it's at 56 psi. Which is right there within +/- 2 psi of the desired 58. Running at anything above idle drops the pressure and at about 3500 rpm it's at about 30 psi.

But turn it off and it settles to 50 psi.

IMG_0391.jpg


And after letting the pressure out and just turning the key on, not running, it went back to 50 psi, not 40 like the first time.

So, anyway... If running at idle is where I'm supposed to see 58 psi then I guess the fuel pump is okay. But I'm not at all sure that it shouldn't be showing that same 58 psi with the key on and not running?

Reason I'm checking... Getting an off and on P0171, lean bank 1. Can't find an exhaust leak. Or a vacuum leak (yet...). Could be fuel pressure if it's neither of those.

Thoughts?

- DAA
 

DAA

Premium Member
Supporting Member
I replaced all four o2s with new OEM about 15K miles ago. Still could be, but it's pretty low on my list of suspects right now.

Google on fuel pressure, not specific to this motor, makes it sound like the static pressure should be the same as idle pressure, and that pressure might actually supposed to go up under load. But none of that is Jeep 4.0 specific. If it does apply to my motor, I think it does have a fuel pressure issue.

Fuel pump is kind of spendy and kind of a PITA so I'm not in too much of a hurry to order one. But if it does need one, it does. I'm just not sure though...

- DAA
 

Hickey

The Dark One
Supporting Member
I replaced all four o2s with new OEM about 15K miles ago. Still could be, but it's pretty low on my list of suspects right now.

Google on fuel pressure, not specific to this motor, makes it sound like the static pressure should be the same as idle pressure, and that pressure might actually supposed to go up under load. But none of that is Jeep 4.0 specific. If it does apply to my motor, I think it does have a fuel pressure issue.

Fuel pump is kind of spendy and kind of a PITA so I'm not in too much of a hurry to order one. But if it does need one, it does. I'm just not sure though...

- DAA
Does it have a fuel pressure regulator?
 

RockChucker

English is important. Engineering is importanter.
Location
Highland
Does it have a fuel pressure regulator?
Jeeps fuel pressure regulators are built into the sending unit, along with the filter.
That does seems like low pressure at higher rpms. I know mine stays pegged at 60 no matter what....but my fuel system is entirely a different beast. I’m not sure what a normal healthy 4.0 fuel system does.
 

glockman

I hate Jeeps
Location
Pleasant Grove
Mine goes right to 55 psi on the gauge at the rail with KOEO. It dips for a second when you start it then sits right at 55 at idle. Mine is an 06 so should have the same pump and fuel system as yours.
Do you have an android and a bluetooth OBD2 reader? Reading O2 voltages is super simple with Torque Pro.
 
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DAA

Premium Member
Supporting Member
I do have Torque Pro, only read the code with it though not the o2 voltage.

CEL light went off while I was testing pressure. Has come and gone like that a couple times recently.

Does your pressure go down with throttle open?

- DAA
 

glockman

I hate Jeeps
Location
Pleasant Grove
I do have Torque Pro, only read the code with it though not the o2 voltage.

CEL light went off while I was testing pressure. Has come and gone like that a couple times recently.

Does your pressure go down with throttle open?

- DAA
I haven't checked it under load but it doesn't move when I rev it in park. I was going to setup a go pro under the hood and go for a romp to ensure the pump can feed the V8. I'll try and do that this weekend and let you know.
I'd say 40 is too low but I'm no expert.
 
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TurboMinivan

Still plays with cars
Location
Lehi, UT
On a non-pressure referenced system such as this, the fuel pressure needs to remain static at all times. If you’re seeing a drop to 30 psi, you definitely have an issue. Clogged filter? Dying pump? Faulty regulator? I’d say one of these is your culprit.


FYI: on a pressure-referenced fuel system such as my turbo Mopars, there is a pressure regulator on the fuel rail. This regulator has a vacuum line attached to it. It has a static setting of 55 psi. Under vacuum, the pressure drops (in this case, by about 1 psi for every inch of vacuum in the intake manifold). The reason for a pressure-referenced fuel system is to add extra fuel capacity when under boost. For that reason, the fuel pressure rises in step with boost pressure—one psi per psi. So at 12 pounds of boost, I’m up to 67 psi of fuel pressure at the injectors... and thus I have more fuel flow than I would at the static 55 psi of fuel pressure.
 
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DAA

Premium Member
Supporting Member
If you’re seeing a drop to 30 psi, you definitely have an issue. Clogged filter? Dying pump? Faulty regulator? I’d say one of these is your culprit.
All three more or less the same repair - replace fuel pump module. Would hate to have it really croak at an inopportune time. Better safe than sorry and all that. Reckon I'll get one ordered.

- DAA
 

UNSTUCK

But stuck more often.
The pressure is way too low at high rpm. This Is actually one of those times where the O2 is doing it’s job well. You’re running lean and if you continued to run it that way you can destroy your engine. Make sure you are getting good voltage to the pump all the time. If you are then replace the pump.
 

DAA

Premium Member
Supporting Member
New Bosch pump module is ordered. Be a couple weeks before I can get to it though. Working on someone else vehicle today, family Moab trip next weekend (in Wife's Jeep). Maybe I can run some of the gas out of the (32 gallon) tank in the meantime :cool: .

- DAA
 

MikeGyver

UtahWeld.com
Location
Arem
There are 2 basic types of fuel systems.

1) Has a regulator on the fuel rail. The idea is it is maintaining a constant pressure ACROSS the injector. If the outlet end is at atmospheric pressure, then you need 58psi fuel at the inlet. If the outlet is under manifold vacuum, then you need less than 58psi fuel to achieve a 58psi delta. This constant allows the computer to calculate fuel flow through the injectors fixed-size orifice by knowing constants and pulse width only. Pretty clever and simple, right?

2) Regulator is usually inside of the fuel tank. Maintains a constant fuel line and rail pressure at all times (58psi, aka 4bar). In order to handle the now varying pressure delta across the injector as the manifold air pressure changes, a curve is programmed in the computer that tells what the injector size "acts like" for a given manifold air pressure. Fueling can then be calculated at every instant. A curve is required because the flow rate through a fixed orifice is not a linear function, as one might assume, This type of system is typically more modern. Below is a screenshot of the curve from my Z06.
Untitled.png


Just to be clear, fuel pressure has nothing to do with RPM, it has to do with manifold air pressure (which can vary independently from RPM).
When you free rev the engine, a sudden brief load is introduced (the acceleration of the rotating mass), but the load drops back down to nearly idle-level once acceleration zeros then goes negative at a new constant RPM. In this example, load is essentially manifold air pressure.
 
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I Lean

Mbryson's hairdresser
Supporting Vendor
Location
Utah
New Bosch pump module is ordered. Be a couple weeks before I can get to it though. Working on someone else vehicle today, family Moab trip next weekend (in Wife's Jeep). Maybe I can run some of the gas out of the (32 gallon) tank in the meantime :cool: .

- DAA
Did the new fuel pump resolve your issues?
 

DAA

Premium Member
Supporting Member
I haven't hooked up the pressure tester again yet, but it hasn't thrown another CEL and seems to have more power at WOT. I will test the pressure again as soon as I can get to it though - I'm curious.

- DAA
 

lhracing

Active Member
Location
Layton, UT
MAP sensor?

The old Edge Trail Jammer for the 4.0 would modify this signal for a performance improvement, I am not sure if it was done by upping the fuel pressure or injector time.

The regulator on the fuel rail may also be dumping too much fuel back the the tank.
 
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DAA

Premium Member
Supporting Member
Lane, this one doesn't have a regulator on the fuel rail or a fuel return line. The regulator is part of the pump module in the tank and the rail only has a damper (it doesn't even have a test port...). I don't believe it's pulse modulated either.

An erroneous MAP sensor reading definitely could cause a lean condition. I think my fuel pressure was low though, definitely wasn't maintaining the static pressure spec anyway. I will get the pressure gauge back on it when I can and see what it looks like now. If it's maintaining pressure and the CEL lean code doesn't come back, I think it will be safe to say it was something in the pump module, whether the regulator, pump itself or even the filter for that matter - they're all part of the module we replaced.

- DAA
 
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