2003 4.0 sohc p0172 & p0175

[cosmicwolf189]

How's a going everyone, I'm new here and made an account so I could potentially get help for an issue with my truck. She's an 03 FX4 Off-Road w Auto trans that's got 222k miles on the clock (bought her at 151k) and for a while now I've been dealing with various trouble codes on the motor. Originally lean codes for both banks which after fixing them are now rich codes for both banks. Overall the engine/fuel parts that I've replaced so far over my 5 years of owning her have been the maf(2018 & 2021), 02 sensors (November 2021), pcv valve (October 2021), intake gaskets (February 2022), the notorious pcv elbow (November 2021), thermostat housing (2021) and fuel filter (2018). As far as I know I believe the plugs/wires are original and she idles in the 700rpm range like normal. Also every now and then when at a stop and shifting from drive to neutral or reverse the rpms drop to around 500 and lights flicker like she's gonna stall out before returning to normal. Anyone have any ideas by chance?

 

[DILLARD000]

Welcome to the Herd!
Lots of friendly knowledgable folk in these forums.

p0172 = RightBank1 Cylinder1+2+3 Too Rich
p0175 = LeftBank2 Cylinder4+5+6 Too Rich
Plugs+Wires along with OxSensors should be renewed every 100kMiles\12yrs.
What Brand OxSensors did you install; best to use Motorcraft\Bosch OxSensors; offbrands tend to die young after a few weeks\months?
Stalling problem; may be IACValve or ThrottlePosSensor or Transmission issue?
How long since a TransmissionService: PanDrop + ValveBodyBoltRetorq + New Filter + New PanGasket + BandAdjust + New 5~7qtsMerconV ?
Click here for more info on V6~4.0L~SOHC Plugs+Wires+CoilPack.
Click here for more info on V6~4.0L~SOHC OxSensors.
Click here for more info on V6~4.0L~SOHC IACValve.
Click here for more info on V6~4.0L~SOHC ThrottlePosSensor.
Click here for more info on 5r55e Transmission.

 

[cosmicwolf189]

Welcome to the Herd!
Lots of friendly knowledgable folk in these forums.

p0172 = RightBank1 Cylinder1+2+3 Too Rich
p0175 = LeftBank2 Cylinder4+5+6 Too Rich
Plugs+Wires along with OxSensors should be renewed every 100kMiles\12yrs.
What Brand OxSensors did you install; best to use Motorcraft\Bosch OxSensors; offbrands tend to die young after a few weeks\months?
Stalling problem; may be IACValve or ThrottlePosSensor or Transmission issue?
How long since a TransmissionService: PanDrop + ValveBodyBoltRetorq + New Filter + New PanGasket + BandAdjust + New 5~7qtsMerconV ?
Click here for more info on V6~4.0L~SOHC Plugs+Wires+CoilPack.
Click here for more info on V6~4.0L~SOHC OxSensors.
Click here for more info on V6~4.0L~SOHC IACValve.
Click here for more info on V6~4.0L~SOHC ThrottlePosSensor.
Click here for more info on 5r55e Transmission.

They're Bosch 02 sensors, I cleaned the IAC which didn't help. I'm pretty sure the issue where it almost stalls isn't trans related even though as far as I'm aware the trans has never been serviced and I know I've never touched it. I'm leaning more towards fuel pump or fuel pressure damper just because it's both banks but I'm still not sure.

 

[RonD]

Fuel pressure damper leak would cause Rich codes, P0172 and P0175
Easy to check if its vacuum hose has fuel in it, it shouldn't of course

Leaking fuel injectors can cause same codes
After engine is warmed up, shut it off
Pull off the air tube from intake
Open throttle and blow into intake then smell the air, if you smell gasoline there IS a leak
When you shut off the engine the fuel injectors are shut off instantly, then RPMs drop to 0, so any remaining fuel smell would be sucked out and GONE
So if you smell gasoline then its leaking in after engine was shut off, warm engine quickly vaporizes gas so easy to smell it


2001-2011 has the fuel pressure regulator in the gas tank with the pump
It is connected to the Fuel Filter by the 3rd port, so its a Return line, even though its called a "Returnless" system, lol
Only 1998-2000 were Returnless, but it didn't work well

Anyway, its hard to test fuel pressure on 4.0l SOHC since most didn't have a pressure test port, you can add one

Picture here: https://cimg4.ibsrv.net/gimg/www.evolutionm.net-vbulletin/2000x1504/1_345281927677f9c03772c2536cc9c1689b88b0f7.jpg

On the lower left are fuel rail end caps, no test port just seals the end
Lower right are the schrader valve test ports
These can be swapped, so you have a test port

Fuel pumps are capable of 80+ PSI
The fuel pressure regulator(FPR) in these years sets pressure at 55-65psi
So failing FPR could cause Rich codes but longshot

 

[cosmicwolf189]

Fuel pressure damper leak would cause Rich codes, P0172 and P0175
Easy to check if its vacuum hose has fuel in it, it shouldn't of course

Leaking fuel injectors can cause same codes
After engine is warmed up, shut it off
Pull off the air tube from intake
Open throttle and blow into intake then smell the air, if you smell gasoline there IS a leak
When you shut off the engine the fuel injectors are shut off instantly, then RPMs drop to 0, so any remaining fuel smell would be sucked out and GONE
So if you smell gasoline then its leaking in after engine was shut off, warm engine quickly vaporizes gas so easy to smell it


2001-2011 has the fuel pressure regulator in the gas tank with the pump
It is connected to the Fuel Filter by the 3rd port, so its a Return line, even though its called a "Returnless" system, lol
Only 1998-2000 were Returnless, but it didn't work well

Anyway, its hard to test fuel pressure on 4.0l SOHC since most didn't have a pressure test port, you can add one

Picture here: https://cimg4.ibsrv.net/gimg/www.evolutionm.net-vbulletin/2000x1504/1_345281927677f9c03772c2536cc9c1689b88b0f7.jpg

On the lower left are fuel rail end caps, no test port just seals the end
Lower right are the schrader valve test ports
These can be swapped, so you have a test port

Fuel pumps are capable of 80+ PSI
The fuel pressure regulator(FPR) in these years sets pressure at 55-65psi
So failing FPR could cause Rich codes but longshot

I did not know about that method for checking leaking injectors so I'll have to try it out! I'm probably gonna look at my fuel damper and injectors here in the next 2 days once all this rain passes.