Intermittent Idle/Bog - JeepForum.com
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post #1 of 22 Old 06-29-2020, 08:06 AM Thread Starter
azhang253
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Intermittent Idle/Bog

Hi,
I have a 2000 XJ with the federal emissions package (2 O2 sensors).

Sometimes, but now always, when idling at a stop light, I will get a very rough idle, where I need to keep my foot on the gas to keep it from stalling.

When I give it gas during the fits of rough idle, there is a very, very sluggish throttle response, and it will surge/pulse.

Also, in general, when idling RPM's fluctuate slightly, maybe 50-60 rpm. The Jeep will idle fine all day, and randomly start acting up.

Here's what I've done to try to diagnose it so far:
  • NEW IAC
  • New Mopar TPS
  • New NTK Upstream O2 sensor
  • Cleaned throttle body
  • New NTK copper plugs
  • New TUPY cylinder head (had this problem before and after the new cylinder head)
  • Smoke tested for Vacuum leaks - no leaks
  • Fuel Pressure Test - Fuel pressure tested good during bad idle
  • New Coil Rail
  • Multi-meter tested all grounds, battery terminals are new, soldered on copper lugs on cables.
  • Tested good compression


Looking at my scanner when running, I'm not seeing any weird MAP values, so I'm assuming my MAP sensor is good. I don't know how to test the coil rail.

I don't think it's a vapor lock issue, since I don't have a problem restarting the jeep after shutting it off, and I don't have any pre-cats with the federal emissions package. Also, this idling issue can happen with the hood completely open.

The only codes I have is for a bad downstream O2 sensor and missing battery temp sensor. I don't believe either of those have anything to do with how the engine runs.


Fuel Trims:
  • I did notice that I have some rich fuel trims during these fits. I'll attach a picture of these
    • LTFT- 33.59
    • STFT- -27.34
  • Due to the fuel trims, I'm thinking one or more of my injectors are intermittently getting stuck open, however; I'm not sure how to test that.


Does anyone have any idea?

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post #2 of 22 Old 06-29-2020, 08:27 AM
jtec
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you have -33 fuel trims AND no codes?

I would recheck codes, current and pendiing, fuel pressure post PSI, and a fuel pressure leak down test, post o2 sensor readings B1S1 and B2S1 if applicable.
Can you see ECT on the scanner? something is AFU with those Ft's.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 22 Old 06-29-2020, 09:11 AM Thread Starter
azhang253
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Quote:
Originally Posted by jtec View Post
you have -33 fuel trims AND no codes?

I would recheck codes, current and pendiing, fuel pressure post PSI, and a fuel pressure leak down test, post o2 sensor readings B1S1 and B2S1 if applicable.
Can you see ECT on the scanner? something is AFU with those Ft's.
Fuel Pressure Tested Good
  • Fluctuating 48-50 at idle
  • Steady at 50 under load

There are no codes other than:
  • Downstream O2 Voltage
    • It's my understanding that the downstream O2 is unrelated to engine function
  • Battery Temperature (the sender is missing)

These trims were taken only during the intermittent bog, not during regular running. I can "accelerate" out of this bog.
I think the ECT reading is a couple boxes up on the attached image. I will try to get the 02 readings while the bog is happening; it may take a bit since it doesn't happen all the time.

Thanks for the reply jtec!

Last edited by azhang253; 06-29-2020 at 09:51 AM.
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post #4 of 22 Old 06-29-2020, 10:36 AM
jtec
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some reason (my skill level?) I cannot blow up the diagram you posted above...

I want to dbl check that there was a - (neg) in front of those fuel trims.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #5 of 22 Old 06-29-2020, 11:12 AM
CJ7-Tim
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You can't really diagnose properly with existing defects. Fix the sensor issues. There might be short circuits causing data corruption.

Wide open throttle acceleration causes the PCM to go to open loop, and all the OBD sensors are ignored.

.
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post #6 of 22 Old 06-29-2020, 01:46 PM Thread Starter
azhang253
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Quote:
Originally Posted by jtec View Post
some reason (my skill level?) I cannot blow up the diagram you posted above...

I want to dbl check that there was a - (neg) in front of those fuel trims.
Jtec, hopefully these images will be more legible for you, and yes, there is a negative. I'm going to try to attach the images to this post.

CJ7-Tim: I will try to find a spare battery temp-thing to eliminate that code.
Attached Thumbnails
P1492 Chrysler.png   P0138 Chrysler.png   XJ PIDs_during_bog.jpg  
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post #7 of 22 Old 06-29-2020, 02:06 PM
jtec
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CJ7-tim is sharp - always right, need to correct the o2 sensor code; What year and models exactly you need to check, I cant remember, BUT jeep started using the post cat sensor to help with fuel trims, and 2002 is a year that utilizes the post cat sensor.

I am confused by the scanner reports - o2 voltages should vary .04 to .07 volts and the information on fuel trims seams to be min and max #s - min max or the actual reading IDK.

back later..

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #8 of 22 Old 06-29-2020, 02:19 PM Thread Starter
azhang253
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Quote:
Originally Posted by jtec View Post
CJ7-tim is sharp - always right, need to correct the o2 sensor code; What year and models exactly you need to check, I cant remember, BUT jeep started using the post cat sensor to help with fuel trims, and 2002 is a year that utilizes the post cat sensor.

I am confused by the scanner reports - o2 voltages should vary .04 to .07 volts and the information on fuel trims seams to be min and max #s - min max or the actual reading IDK.

back later..
I will try to pull some more values. My scanner is a USB one that plugs into my laptop. There's a column for current value, Min, Mean, and Max for each PID. It's just a little difficult logistically to grab the values while driving around since it's on my laptop.
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post #9 of 22 Old 06-29-2020, 04:32 PM Thread Starter
azhang253
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OK, I've replaced my downstream O2 sensor and plugged in the battery sensor. The battery sensor doesn't seem to work.

After plugging in the downstream O2, I've gotten a bunch of new info and a bunch of weird ****.

1. Both O2 sensors are now showing short to voltage!?? But they were just reading...
2. Cylinder 2 misfire.
Attached Thumbnails
pids post o2 sensor.jpg  
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post #10 of 22 Old 06-29-2020, 05:31 PM
CJ7-Tim
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All of the trouble codes mention open circuits or short circuits as a possible cause of the codes. The OBD-II engine sensors are all on a 5 volt circuit. If there is 12 volts shorted to this 5 volt circuit the OBD data is corrupted.

Loose O2 wires often get chafed insulation or get damaged from rubbing on the driveshaft.

.
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post #11 of 22 Old 06-29-2020, 08:02 PM Thread Starter
azhang253
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Quote:
Originally Posted by CJ7-Tim View Post
All of the trouble codes mention open circuits or short circuits as a possible cause of the codes. The OBD-II engine sensors are all on a 5 volt circuit. If there is 12 volts shorted to this 5 volt circuit the OBD data is corrupted.

Loose O2 wires often get chafed insulation or get damaged from rubbing on the driveshaft.

I didn't see any chafed wires, but I will look more tomorrow. It's difficult to see the wires above the transmission.

I DID however, find a blown fuse, which I've labeled in the photo below. After replacing the fuse, there was no change in my fluctuating idle, however; my scan tool picked up my O2 sensors again.

After changing the fuse, my PIDs look good again.

Looking at wire diagrams, it looks like Fuse 17. The fused circuit is F99 18RD (Fused Circuit 99, 18 gauge RED). Lo and behold, F99 18RD is involved in both O2 sensors, in cavities C16 and C11 at PDC.

Judging by wiring diagrams, F99 18RD, starts at the PDC. At the PDC: Fuse 17-->Downstream Relay--> Upstream relay.

From the relay, I will be looking for A42 18DG (Dark Green) to the Upstream O2 and A242 18VT/OR (Violet/Orange) to the Downstream O2. Both of these pass through Connector 107.

After the O2 sensor, both black wires terminate at G101 (the two grounds on the passenger side of engine), passing through splice 107.

My brain is fried, I'm going to hunt down these wires tomorrow when I have more daylight and less alcohol. According to my understanding, there is likely to some kind of wire damage in one of the above wires that can blow the fuse, right?
Attached Thumbnails
IMG_4117.jpg   fuse label.png   PDC fuse box.png   pdc o2 relay to O2.jpg  
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post #12 of 22 Old 06-29-2020, 08:23 PM Thread Starter
azhang253
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After replacing the fuse, I was idled for around 20 minutes. No change in idle, same fluctuations in RPMs. I wasn't able to reproduce the bogging, but it's always been intermittent.

Here are the pids that I was able to pull.

Edit: imgur link, these pictures get resized all weird when I upload them here. https://i.imgur.com/RixuhJr.png

Testing for Tomorrow:
TESTING:
1. Check for continuity between C107 pins 7 and 13 with ground

2. Check for continuity between Upstream O2 sensor A42 (Dark Green) and ground, as well as Downstream O2 sensor A242 (Violet/Orange) and ground

3. Do my best to find any problems with the harness between C107 to the O2 Sensors
Attached Thumbnails
new readings post blwon fuse.jpg  

Last edited by azhang253; 06-29-2020 at 10:34 PM.
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post #13 of 22 Old 06-30-2020, 08:13 AM
CJ7-Tim
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Quote:
Originally Posted by azhang253 View Post
..... when idling RPM's fluctuate slightly, maybe 50-60 rpm.
This is likely unrelated to the other symptoms. I would clean the throttle body and consider buying a new genuine Jeep Idle Air Controller.

.
“When the people find that they can vote themselves money, that will herald the end of the Republic.”

- Benjamin Franklin
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post #14 of 22 Old 06-30-2020, 10:17 PM Thread Starter
azhang253
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Update:
  1. Using my multimeter, I ensure that none of the pins at the PDC for the O2 heater circuit had shorted to ground
  2. I back-probed Cav 7 and 13 on C107 and found no shorts to ground
  3. I visually inspected the O2 sensor harness that runs above the transmission and found no frayed wires. All the OEM wire-protectors/conduit was still in good shape. My O2 sensor pigtails are band new (since the sensors are brand new). I didn't want to drop the transmission to take apart the harness.
  4. I back-probed Cav 1 on both O2 sensors and:
    • Found no short-to-grounds, no matter how I wiggled the harness
    • With the vehicle running, I got 13.9V. This stayed constant no matter how I poked, prodded, and wiggled the harness.
  5. At this point, I concluded that, at the time of testing, there is no problem with the harness.
So, why did my fuse blow???

My hypothesis is this:
I know the old downstream O2 sensor was bad. We know, as shown in the wiring diagrams above, the downstream O2 sensor heater circuit is directly connected to Fuse 17.
When changing the O2 sensor, I did NOT remove the negative battery terminal. I'm pretty sure I had the key off, but not 100% sure. There may have been an internal short in the sensor that caused the fuse to blow.

Today, I idled the vehicle for a total of about an hour, with no problems and no blown fuses. I haven't had a chance to drive it around yet (I work from home, at least during COVID, and didn't need to drive anywhere).

ALSO, the FSM specifically notates "Never apply any type of grease to the oxygen sensor electrical connector, or attempt any soldering of the sensor wiring harness. For sensor operation, it must have a comparison source of oxygen from outside the exhaust system. This fresh air is supplied to the sensor through its pigtail wiring harness." Well, I did apply dielectric grease when reinstalling, so I cleaned all that out with some electrical contact cleaner. For ****s and giggles, I cleaned out both O2 sensor connectors and C107.

After cleaning the connectors and re-zip tying everything, idling for around 20 minutes, I noticed that my CEL had turned off! I plugged in my scanner, and all O2 sensor codes are now gone, as well as the battery temp. I was able to idle multiple times for around 20 minutes with no problems. The small idle fluctuations still exists. I took apart my throttle body and super-cleaned it, including the IAC hole, but it made no difference. My fuel trims looked pretty normal.


Lastly, after ALL this, I see the following in the FSM:
"Downstream Sensor (Non-California Emissions):
The downstream heated oxygen sensor (1/2
sensor) is located near the outlet end of the catalytic
convertor. The downstream sensor is also used to
determine the correct air fuel ratio. As the oxygen
content changes at the downstream the PCM calculates
how much air fuel ratio change is required. The
PCM then looks at the upstream oxygen sensor voltage
and changes fuel delivery until the upstream
sensor voltage changes enough to correct the downstream
sensor voltage (oxygen content).
The downstream oxygen sensor also provides an
input to determine catalyst efficiency."


Apparently, the downstream O2 IS used in determining the air-fuel ratio!????? Also, now knowing that the downstream and upstream O2 sensors are on the same circuit, it makes sense that problems with the downstream can also cause problems with the upstream O2 sensor.

So, I'm hopeful (but not holding my breath) that this was my issue. On Friday, I will be going on a long road trip, ~400 miles with probably 30 miles of 2 tracks to the Pictured Rocks in Upper Michigan, so we'll definitely know if the problems come back. I'm not too worried, since I made this trip in December, with all the same problems and no new sensors/iac/coil.
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6-30-2020_pids.jpg   6_30_2020 zero dtc.jpg  
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post #15 of 22 Old 07-01-2020, 07:19 AM
monkeyhouse
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Awesome

What an informational thread! Great read, thanks!

First Jeep was an 1989 YJ, I've also had a Grand Cherokee. Currently, I have a 99 XJ.
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