2004 4.0 LJ Auto - Fuel Rail Pressure questions.

[4x4 AG]

I have been chasing a low idle issue, and well I have been around the block a few times. Great thread on here looking for vacuum leaks and fuel trims diagnosis.

Well it still does it, and now I have made it all the way to testing the fuel pump, thinking maybe it is weak and that is accentuated at the low RPM idles.

Well I have a 2004 with no shrader, so finally built a contraption to test it. Here is what I found.

Priming the Key, it goes to 40psi. that is highest.

Idling: needle bounces, but around 45-46psi.

Rev it in park: It stops bouncing and levels at 46psi.

I drove it around, and got it warm. came back.

All pressures still the same.

Chocked the tires and set brake, Put in Drive and at idle it is 45-46psi.

I cut off the engine and it dropped and held at 40psi.

Heres my questions:
1. does all this sound normal?
2. per the service manual, I think is says the 2004 is supposed to be Fuel specification is 400 KPa +/- 34 KPa (58 psi +/- 5 psi), but i thought is was lower than that in other threads i have seen.

I just want to know if this could be a potential issue, as dropping the tank is an ordeal.

 

[4x4 AG]

ok i did find this reading the Service Manual.

The regulator is calibrated to maintain fuel system operating pressure of approximately 339 ± 34 kPa (49.2 ± 5 psi) at the fuel injectors.

so maybe it is good?

 

[GATORB8]

 

[GoldenGorilla]

ok i did find this reading the Service Manual.

The regulator is calibrated to maintain fuel system operating pressure of approximately 339 ± 34 kPa (49.2 ± 5 psi) at the fuel injectors.

so maybe it is good?

I went back to your original post. Your idle RPM was 655. This is close to normal. The idle in Drive is 648 when the engine coolant temp is between 185F and 212F and the idle in park is 664 in the same conditions.

I do not see that you have a problem.

 

[macleanflood]

My fuel pressure was as low as 35 psi in my 97.

When monitoring live ODB data with a $25 ODB Bluetooth dongle and the free app Torque Lite I could see double digit fuel trims and my gas mileage plummeted...

Swapped to the almost cheapest pump assembly from Rock Auto...a Brute Power...and my fuel pressure is back to 49, gas mileage is up and fuel trims are low single digits.

Racetronix makes a 5/16 QD to 1/8" T (link in my profile) that will let you hang a fuel pressure gauge off the side of the fuel rail. I have one permanently mounted for quick diagnostics... although high under hood heat and two years, Moab, two Rubicon trips, Canada and back the glass is cracked and the oil is leaking out... probably going to switch it to a digital glowshift pressure gauge and run it into the cab next to my glowshift oil pressure gauge.

-Mac

 

[4x4 AG]

I went back to your original post. Your idle RPM was 655. This is close to normal. The idle in Drive is 648 when the engine coolant temp is between 185F and 212F and the idle in park is 664 in the same conditions.

I do not see that you have a problem.

I would like to agree, and hope you are right. I just feel like the idle being low, causes a slight vibration and rattle in some of the interior plastics when it drops down.

I will agree, nothing seems glaringly wrong.

Last night we pulled apart everything on the EVAP system, removed and pluged the filter line, and smoked it. NO smoke anywhere. removed the gas cap to insure full system was smoked and it was. so nothing there.

Today I smoked the engine again. only spots anything appears, soon was at the butterfly shaft on the Thottle body, after quiet some time, at the front PCV hose to the intake. I zip tied it tighter. Once i determined nothing more. I started the engine, and sprayed starting fliud anywhere i saw smoke. Even directly on the butterfly spots that leaked. No change in idle, so i assume it isnt bad enough.

 

[4x4 AG]

I went back to your original post. Your idle RPM was 655. This is close to normal. The idle in Drive is 648 when the engine coolant temp is between 185F and 212F and the idle in park is 664 in the same conditions.

I do not see that you have a problem.

I would like to agree, and hope you are right. I just feel like the idle being low, causes a slight vibration and rattle in some of the interior plastics when it drops down.

I will agree, nothing seems glaringly wrong.

Last night we pulled apart everything on the EVAP system, removed and pluged the filter line, and smoked it. NO smoke anywhere. removed the gas cap to insure full system was smoked and it was. so nothing there.

Today I smoked the engine again. only spots anything appears, soon was at the butterfly shaft on the Thottle body, after quiet some time, at the front PCV hose to the intake. I zip tied it tighter. Once i determined nothing more. I started the engine, and sprayed starting fliud anywhere i saw smoke. Even directly on the butterfly spots that leaked. No change in idle, so i assume it isnt bad enough.

 

[4x4 AG]

My fuel pressure was as low as 35 psi in my 97.

When monitoring live ODB data with a $25 ODB Bluetooth dongle and the free app Torque Lite I could see double digit fuel trims and my gas mileage plummeted...

Swapped to the almost cheapest pump assembly from Rock Auto...a Brute Power...and my fuel pressure is back to 49, gas mileage is up and fuel trims are low single digits.

Racetronix makes a 5/16 QD to 1/8" T (link in my profile) that will let you hang a fuel pressure gauge off the side of the fuel rail. I have one permanently mounted for quick diagnostics... although high under hood heat and two years, Moab, two Rubicon trips, Canada and back the glass is cracked and the oil is leaking out... probably going to switch it to a digital glowshift pressure gauge and run it into the cab next to my glowshift oil pressure gauge.

-Mac

i have thought about putting one on there. Maybe in the future, but mainly trying to trace down if there is anything i need to fix prior to putting my 16 yo in this jeep as his daily commuter.

 

[hear]

My fuel pressure was as low as 35 psi in my 97.

When monitoring live ODB data with a $25 ODB Bluetooth dongle and the free app Torque Lite I could see double digit fuel trims and my gas mileage plummeted...

Swapped to the almost cheapest pump assembly from Rock Auto...a Brute Power...and my fuel pressure is back to 49, gas mileage is up and fuel trims are low single digits.

Racetronix makes a 5/16 QD to 1/8" T (link in my profile) that will let you hang a fuel pressure gauge off the side of the fuel rail. I have one permanently mounted for quick diagnostics... although high under hood heat and two years, Moab, two Rubicon trips, Canada and back the glass is cracked and the oil is leaking out... probably going to switch it to a digital glowshift pressure gauge and run it into the cab next to my glowshift oil pressure gauge.

-Mac

Very likely I'm going to do the same. I'm sure Glowshift is no Autometer, but for my purposes it's perfectly fine and in some ways better (ie the color changing, whatever, don't @ me). And the reviews were somewhat mixed but I've installed several and am pretty confident that if you secure the gauge well and don't wire it like a caveman it will be a reliable unit. I've had the one in my 97 mounted for going on 4 years, and zero issues.