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Unread 02-11-2013, 06:32 PM   #1
donthelegend
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P0171 Causes

Backstory:

Bought a '97 XJ with a blown head gasket. It would barely run so I basically drove it up and down the street before purchasing and that was about it.

Had a buddy who was parting out a '98 with a good motor and recently rebuilt transmission, so I bought the '98 drivetrain from him. Before swapping it in, I replaced the RMS, oil pan gasket, OFA o-rings, valve cover gasket, and swapped the transmission tailhousing, sensor drive rotor, and output speed sensor from the '97 onto the rebuilt trans. had to swap a few other sensors from the '97 to the '98 as well so the plugs matched (camshaft position sensor, oil pressure sensor, and coil specifically). Also installed an APN header to fix the cracked factory one. Cleaned the throttle body and IAC, new plugs (champion coppers), wires, cap and rotor. All new hoses, waterpump, thermostat, air filter, etc.

So all tune-up parts are fresh.

The motor looked great internally, no sludge at all in the valve train nor the oil pan.

Finally get the good drivetrain swapped in, fresh fluids, etc. On initial startup it smoked for a while, but while replacing seals it was upside down on the engine stand for a while so I didn't really worry about it. After driving it around the block it cleared up and has been fine. Oil pressure is good, a little over 20 PSI at hot idle. It idles well at a little over 700 RPM for a while, but once up to temperature it starts to idle a little rough. Not terrible, but it fluctuates and there is a noticeable difference in the smoothness of the idle. Its also throwing code P0171.

I have verified fuel pressure at the rail at roughly 47 PSI, which is in spec (barely, but it holds rock solid at 47 and never even hints at dropping so I don't think this is the problem).

Upstream 02 seems to be another common failure. Had a spare O2 of unknown condition (was pulled from a running vehicle but I don't know its history) and still have the same code, although it appears even faster now. With the original O2 it would take a little while for the code to show up, now it appears immediately. It comes back as soon as I check for codes again after clearing it. Now its possible that both sensors are bad, but I want to add some other info first.

Watching my fuel trims while running, they look way out of whack, and they are fluctuating like crazy, but I don't know what that points to. STFT will stay around 7 and then jump to 20-30, stay there for a while, then jump back down, sometimes into the negatives. LTFT is different but equally crazy. Won't settle down at all. I think this is probably key to my next step but like I said, I'm not sure what that points to.

I've read that vacuum leaks can be a problem, and wouldn't be unheard of since I just did a motor swap, but I haven't found any yet, and it idles really nicely until warm so I don't think thats it either.


Sorry for the book. If you're still reading, thanks and let me know what you think!

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Unread 02-11-2013, 06:51 PM   #2
CJ7-Tim
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P0171 JEEP - Fuel System 1/1 Lean

Possible causes
- Intake manifold leaks
- Exhaust manifold leaks
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Incorrect fuel pressure
- Lack of fuel
- Incorrect Positive Crankcase Ventilation (PCV) hose connection
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Unread 02-11-2013, 07:21 PM   #3
donthelegend
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Quote:
Originally Posted by CJ7-Tim View Post
P0171 JEEP - Fuel System 1/1 Lean

Possible causes
- Intake manifold leaks
- Exhaust manifold leaks
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Incorrect fuel pressure
- Lack of fuel
- Incorrect Positive Crankcase Ventilation (PCV) hose connection
Thank you, just what I was looking for!

I've eliminated intake and exhaust manifold leaks (although a vacuum leak someplace else is still a possibility, I've checked the commonly damaged lines at the vacuum canister and cruise control)

Front O2 sensor has not been eliminated, but a second O2 sensor was tried with no change.

Ignition misfire is possible, although I'm not getting a misfire code.

Faulty fuel injector is also possible. I ran a bottle of fuel injector cleaner through the 5 or 6 gallons of gas that was left in the tank. Also thought bad/old fuel could be contributing so I ran it down to fairly empty and added fresh gas tonight, will drive it about 50 miles tomorrow so we'll see if that does anything.

Verified fuel pressure already so thats out.

Lack of fuel is out as well, but see above about possibly bad fuel. This jeep has sat with the same gas for over a month and I'm not sure how old the fuel was when I bought it. Should find out tomorrow if this was the problem.

PCV connections are correct so thats out as well.



Any thoughts on the crazy fluctuations in the fuel trims? I don't know if the trims by themselves actually mean anything, it just seems strange that they are all over the place rather than consistently in some range, even if that range is high enough (or low enough) to throw a code.
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Unread 02-11-2013, 07:36 PM   #4
CJ7-Tim
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You are sure about the exhaust manifold? +80% of the XJ's in the junkyard have a cracked exhaust manifold that sucks in fresh air, which will cause a the O2's to read a lean condtion.

The trims are fluctuating due to the fault. The computer cannot keep up, or trim correctly, due to the fault and throws the code, which basically means it is totally confused.

.
crack-1-.jpg  
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Unread 02-11-2013, 07:43 PM   #5
MJR
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I would flush out and put fresh fuel in it. The misfire monitor becomes inactive after a total reset until it's been driving for a while. If you can get a misfire code it would be easier to swap out cylinder parts (plug, wire, injector) to isolate. Possible TPS as well I would think and if you got a bunch off oil on the O2's they would be suspect. Also I would check to see if your PCM has the latest software in case improvements have been made.
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Unread 02-11-2013, 07:47 PM   #6
CJ7-Tim
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I bought a Cherokee that sat under a tree, with the gas cap off, for 2 years. After repairs, it started right up and drove to the gas station for fresh gas.
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Between 2009 when Obama took office, and 2013, median annual household incomes have fallen by more than $2,100.

Progressive Liberalism: Bringing you new Healthcare ideas so wonderful, they have to include mandatory participation ......

Originally Posted by Ronald W. Reagan: Government is not the solution to our problems; Government is the problem.
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Unread 02-11-2013, 07:48 PM   #7
donthelegend
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Brand new APN header (the manifolds on both the bad engine and good one were cracked as in your picture). Installed the new header while on an engine stand so I know I torqued all those bolts that are such a PITA to get to, and the intake and exhaust manifolds were seated properly.

Looks like at this point I'm going to go over all the vacuum lines again and run some more fuel injector cleaner through it, figure that can't hurt.
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Unread 02-11-2013, 08:04 PM   #8
MJR
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The lastest flash update software fixes these issues, adds improvements, and adds a feature.


Erroneous DTC Hex $24 - MAP SENSOR VOLTAGE TOO LOW
Erroneous DTC Hex $C0 - SLOW UPSTREAM 02S DURING CATALYST MON. or SLOW LEFT BNK UP 02S DURING CATALYST MON.
Erroneous (DTC) of P1763 –Transmission Governor Pressure Sensor Volts Too High.
Erroneous DTC P1398 - No Crank Sensor Learn
Misfire Monitor Improvements
Generic Scan Tool Corrections (Mode 6 Last Monitor Results)
Long Crank Time at High Altitude (>8000 ft.)
Light Spark Knock Timing Retard Feature


5010400AB
50 STATE (EXCEPT CALIF. 4WD) AUTO 4 SPEED

Vehicle Description :
1997 XJ - 4.0L 6 CYL (MPI)

New Part # Flash 05010400AB supersedes :
05010400AA
05014152AA
56041277AA
56041277AB
56041277AC
56041277AD
56041277AE
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Unread 02-11-2013, 08:19 PM   #9
donthelegend
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Little quick to jump to the PCM isn't it? Sure its a possibility but I've got few other things to work through first. If the code comes back while driving tomorrow I'll pick up a new O2 sensor instead of the spare one I had lying around that I put in tonight and go from there. Thanks for the help so far, I'll report back tomorrow night with any new info.
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Unread 02-11-2013, 08:25 PM   #10
MJR
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I wasn't suggesting a PCM only a flash update to the current level of software.
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Unread 02-11-2013, 08:26 PM   #11
CJ7-Tim
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I would be inclined to buy a new NTK/NGK O2 sensors, and to reboot the PCM after the install. A reflash or PCM update should not be needed.

Sorry, I missed the part of the OP about the APN.


This very simple procedure will erase the 1996-2001 “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed.

After performing this procedure the 1996-2001 PCM will re-learn and store into Adaptive Memory your engines performance characteristics.

Please perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn Mode adaptive mode.

Disconnect the POSITIVE battery Terminal and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”


The PCM Adaptive memory has now been flashed, or erased from the PCM.

When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory.

When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”.

The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.
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Between 2009 when Obama took office, and 2013, median annual household incomes have fallen by more than $2,100.

Progressive Liberalism: Bringing you new Healthcare ideas so wonderful, they have to include mandatory participation ......

Originally Posted by Ronald W. Reagan: Government is not the solution to our problems; Government is the problem.
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Unread 02-11-2013, 08:42 PM   #12
donthelegend
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Quote:
Originally Posted by MJR View Post
I wasn't suggesting a PCM only a flash update to the current level of software.
Sorry, for some reason when I saw that post, all it had was the part numbers for PCMs, that top of it was gone.


Quote:
Originally Posted by CJ7-Tim View Post
I would be inclined to buy a new NTK/NGK O2 sensors, and to reboot the PCM after the install. A reflash or PCM update should not be needed.

[snip]
Yep, thats my plan for the next step. Will report back tomorrow.
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Unread 02-11-2013, 08:49 PM   #13
MJR
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Quote:
Originally Posted by donthelegend View Post
Sorry, for some reason when I saw that post, all it had was the part numbers for PCMs, that top of it was gone.

My bad, fixed it.
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Unread 02-14-2013, 09:54 AM   #14
donthelegend
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Just a quick update, looks like the O2 sensor fixed it. Have put a little over 100 miles on it and the fuel trims have stayed in spec and no CEL. Thanks all for the help.
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