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96 XJ stumbles then shuts off when warmed up. P0171

M

Mrpd1501

NAXJA Forum User
Location
Texas
Sorry to bring this up again, I've combed the forums for answers and come up dry.

Bought a beater for my deer lease, a 96 XJ 4.0 (Country edtn, if it matters).
Found a LOT of bad wiring under the hood. (think wire nuts)

Replaced the following before I even thought about taking it out to the lease:

PCM - burned mosfet on existing due to shorted injector wires
- replaced the mosfet on the pcm it came with so i have a spare

Exhaust from the manifold back, including sensors, cat, and muffler - Walker and Bosch (yes i know)

thermostat and housing(flushed and refilled coolant as well)
Plugs
Wires
Dist Cap and Rotor
Ignition coil
Misc lights
Oil, filter

Here's where the fun starts -
Got about two miles into the lease and the engine finally got up to temp.
Next thing you know, rough idle that got worse on applying accelerator, then stall.
Key off 30-45sec, hard crank then start like nothing happened.
A few minutes of run time later, same thing.
Now, it won't start at all.

Replaced yesterday:
TPS
IAC
Coolant temp sensor

I got it up to 208 per the scan tool yesterday, O2 sensor looked like it was reading correctly until it shut down. Only code I got was a P0171.
Fuel pressure at the rail while idling was a solid 30PSI until the stall condition reoccurred, then I couldn't get pressure at all.

Haven't done the CPS yet. Wondering if a faulty CPS would stop the pcm from allowing the fuel pump to push anything at all in the case of unexpected status. (I've got to wait until have another person out there with me to keep watch while I'm under it for safety purposes)

I'll be replacing the MAF this afternoon when I get time to run up there. NTK O2 upstream should be here tomorrow to replace the Bosch I have in there.

Just wondering what else I should look at before I cut a hole in the floor pan to get the fuel pump out the lazy way.

Thanks in advance
 
A faulty CPS would result in no fuel pump operation.

It is well proven that Bosch O2 sensors in a Jeep 4.0L can cause driveability issues.

Most XJ Cherokee 4.0L have a cracked exhaust mainfold.

At 25 years of age, wire harness issue are very common, and a logical/reasonable suspect.


What are the Possible Causes of the P0171 JEEP Code?

Faulty front heated oxygen sensor or wiring
Ignition misfiring
Faulty fuel injectors or wring issues
Exhaust gas leaks
Incorrect fuel pressure
Lack of fuel
Intake air leaks
 
Looking at Tim's advice (good stuff) and considering your experience, I would take a good luck at this detail:

"Faulty front heated oxygen sensor or wiring"

Most particularly, I would look for any place where the wiring for the O2 sensor may have gotten into contact with the exhaust. Given that you just replaced the O2 sensors it is entirely possible that the routing for the wiring has been changed (possibly just on account of stiff, new wires on the new sensor) and allowed to get into contact with the exhaust.

Before attacking the fuel pump I would go after the fuel filter.

On a '96 you don't stand to gain much by cutting a hole in the floor. The fuel pump is accessed from the front of the tank, not the top. You can change the pump without dropping the tank if the fuel level is below the level of the access ring.

If you are going to deal with the CPS I highly recommend slotting the holes on the new CPS. Then carry a spare that also has slotted holes. That can save you the aggravation of a dropped CPS bolt in the bellhousing.
 
Thank you both! I'll get the proper O2 sensor in it tonight and see what changes. Fuel filter is in the back seat as well.

Still stumped on the zero pressure after the last stall yesterday.

I shook the harness while it was running to see if there was any variance in idle speed, nothing at all. It literally runs like a top til it gets above 203, then it goes to sh!t.

Bought this to avoid having to pay market for a side by side right now, but I think I'd have saved money going with the side by side about now. :D
 
I should also add that i sprayed a cloud of carb cleaner around the engine bay at the beginning of the stutter, no increase in idle speed. Zero affect.
 
Add the Coolant Temeparature Sensor on the thermostat housing to your list of sensors to test, and wires to inpsect.
 
Tim_MN said:
Add the Coolant Temeparature Sensor on the thermostat housing to your list of sensors to test, and wires to inpsect.
Click to expand...

I replaced that one, and it's working as far as I know. Coolant temp shows up in the scan tool live data. Is there another temp sensor somewhere? Or is that a relative reading I'm seeing?
 
If you can see the CTS live, then it is probably fine. Heat related malfunctions are very often, cheap Chinese sensors that go out of tolerance when hot, when good quality sensors will continue to send valid and true data even when hot. Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.
 
Last edited:
Tim_MN said:
I you can see the CTS live, then it is probably fine. Heat related malfunctions are very often, cheap Chinese sensors that go out of tolerance when hot, when good quality sensors will continue to send valid and true data even when hot. Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.
Click to expand...

I'm with you there, 100%. When someone offers me a lifetime warranty on something, I ask them if it means my lifetime, the lifetime of the thing the part is attached to, or simply the life of the part. Specificity matters.

Of late, sourcing quality parts is difficult and lead times are incomprehensibly long. My aim here is to pinpoint the cause of the issue, then order parts from the dealer that I know will last.

I don't mind doing the work twice, but sitting idle for six weeks (that's estimate on cps from my local dealer) while I wait for a high dollar part that may or may not fix the issue isn't an attractive option for me.

If it's the CPS (or whatever other part it ends up being) and this cheap part proves it- great, I'll call the jeep dealer and order the absolute correct item and wait for it. In the meantime, I get to make use of my hunting vehicle while the feral hogs are out getting fat. ;)
 
I think the pump should still initially prime even if the cps is crap. I think either your pump is crap or you got a lot of crap in the tank. Someone here recently had a problem similar to you and it turned out debris in the tank was clogging the sock. You can remove your pump without dropping the tank. Be careful not to damage your sending unit assembly though. Some 96s had a 1 year specific unit that is difficult to find.
 
Unclewolverine said:
I think the pump should still initially prime even if the cps is crap. I think either your pump is crap or you got a lot of crap in the tank. Someone here recently had a problem similar to you and it turned out debris in the tank was clogging the sock. You can remove your pump without dropping the tank. Be careful not to damage your sending unit assembly though. Some 96s had a 1 year specific unit that is difficult to find.
Click to expand...

This is a very good point, some of the parts on this engine only come up associated with a 97, it's odd. I thought it was simply because the previous owners threw things together, but I guess there was a mid-year production change for some things under the hood. Either way, this xj has had me pulling my hair out.

I just wanna hunt and fish, mannnnnnnn....
 
1996 is a bastard year for the XJ. Early build 1996's are said to be OBD-I. Numerous changes were almost randomly dropped into the production line all throughout the model year, resulting in the later units being OBD-II. Some people report having to drag their parts to the auto parts store and asking the counter people to bring out a 95 and a 97 part to compare to the original.

As already suggested, fuel delivery issues should be ruled out by inspection and diagnostic testing of the hardware and the wiring.



Below is some CPS info, any non-genuine Jeep CPS is suspect.

CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again when it cools back down.

Typical CPS Symptoms, (not all symptoms may be present, or occur at the same time) -
- Random stalling
- Starter cranks and cranks but engine won't start up
- Fuel gauge and voltage gauges may not work or display properly.
- You sometimes will have NoBus displayed on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- Fuel pump should run and prime for 3-5 seconds.
- No spark at the spark plugs.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.


If the CPS/CKP is failed sometimes the OBD-II code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.

Diagnostic steps to help confirm the CPS is the cause of your no-start:
-You should be able to verify a bad cps, by unplugging it, and turning the ignition key to ON. If the voltage gauge and/or the fuel gauge now displays correctly, replace the CPS.
-Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.
-Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate these relays as the cause of the no-start. Confirm that the fuel pump runs for 3-5 seconds when you turn the ignition key to ON.
-Eliminate the NSS as a cause of no start. Wiggle the shift lever at the same time you try to start. Put the transmission in Neutral and do the same. Do the reverse lights come on when the shifter is in Reverse?
-Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will lose communication.
-Check the ground connection for the PCM. Make sure it is clean, snug, and has no corrosion.

If you buy a new CPS, get a genuine Jeep CPS. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

You must also perform basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU will behave oddly until you remedy this.
Crankshaft Position Sensor Connector (CPS/CKP)
.
CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3 (See Image). Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.
 
Tim_MN said:
1996 is a bastard year for the XJ. Early build 1996's are said to be OBD-I. Numerous changes were almost randomly dropped into the production line all throughout the model year, resulting in the later units being OBD-II. Some people report having to drag their parts to the auto parts store and asking the counter people to bring out a 95 and a 97 part to compare to the original.

As already suggested, fuel delivery issues should be ruled out by inspection and diagnostic testing of the hardware and the wiring.



Below is some CPS info, any non-genuine Jeep CPS is suspect.

CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again when it cools back down.

Typical CPS Symptoms, (not all symptoms may be present, or occur at the same time) -
- Random stalling
- Starter cranks and cranks but engine won't start up
- Fuel gauge and voltage gauges may not work or display properly.
- You sometimes will have NoBus displayed on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- Fuel pump should run and prime for 3-5 seconds.
- No spark at the spark plugs.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.


If the CPS/CKP is failed sometimes the OBD-II code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.

Diagnostic steps to help confirm the CPS is the cause of your no-start:
-You should be able to verify a bad cps, by unplugging it, and turning the ignition key to ON. If the voltage gauge and/or the fuel gauge now displays correctly, replace the CPS.
-Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.
-Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate these relays as the cause of the no-start. Confirm that the fuel pump runs for 3-5 seconds when you turn the ignition key to ON.
-Eliminate the NSS as a cause of no start. Wiggle the shift lever at the same time you try to start. Put the transmission in Neutral and do the same. Do the reverse lights come on when the shifter is in Reverse?
-Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will lose communication.
-Check the ground connection for the PCM. Make sure it is clean, snug, and has no corrosion.

If you buy a new CPS, get a genuine Jeep CPS. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

You must also perform basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU will behave oddly until you remedy this.
Crankshaft Position Sensor Connector (CPS/CKP)
.
CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3 (See Image). Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.
Click to expand...

Funny, I thought all of the communication failures that I neglected to mention were just a result of the random rat nests of wiring found throughout. I'm gonna go grab a neighbor to keep watch while I test that CPS.
 
I could have sworn a 96 should have 49+/-2 psi for the fuel pressure. Single line from the tank without a regulator on the rail, right?

You video shows something isn't holding pressure. As fast as it drops, I'd say either the check valve in the tank isn't working or there's a leak inside the tank between the pump and fuel line.
 
lawsoncl said:
I could have sworn a 96 should have 49+/-2 psi for the fuel pressure. Single line from the tank without a regulator on the rail, right?

You video shows something isn't holding pressure. As fast as it drops, I'd say either the check valve in the tank isn't working or there's a leak inside the tank between the pump and fuel line.
Click to expand...

I thought I read 31 somewhere. It’s pretty constant at 30 otherwise. That gauge is sketchy, not sure how much I trust it.
 
lawsoncl is correct. 49 psi for a '96.

Or, another way to put it, 49 psi unless you have a regulator on the fuel rail.

The earlier years have a regulator on the fuel rail and (I believe) a vacuum signal. The later years (starting with '96) put the regulator in the tank where there is no vacuum signal.

So that is indeed where you need to focus attention.

It might be easiest to pull the fuel filter first and just check that to make sure it isn't plugged. Murphy says your problems will be in the tank, but if you give him that sort of credit up front he will nail you with the fuel filter anyways.
 
Anak said:
lawsoncl is correct. 49 psi for a '96.

Or, another way to put it, 49 psi unless you have a regulator on the fuel rail.

The earlier years have a regulator on the fuel rail and (I believe) a vacuum signal. The later years (starting with '96) put the regulator in the tank where there is no vacuum signal.

So that is indeed where you need to focus attention.

It might be easiest to pull the fuel filter first and just check that to make sure it isn't plugged. Murphy says your problems will be in the tank, but if you give him that sort of credit up front he will nail you with the fuel filter anyways.
Click to expand...

I know Murphy well, he lives on my left shoulder. Had other stuff come up the last two days that kept me from getting back up to where the jeep is. Going tomorrow after work to dig back into this. Gotta look up where this regulator would be if it was present on the rail. Could have sworn i saw 31psi somewhere.
:piratefla
 
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