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  #1  
Old May 17th, 2012, 18:24
stonedv8 stonedv8 is offline
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2000 Cherokee 4WD 4.0 Backfiring through Intake

I searched and found only one other with a similar year model, and his was throwing O2 sensor codes but he never posted again what may have fixed his issue.

This is/was my daily driver so I am hitching a ride with the wife until I can resolve this. Adds about 3 hrs to my schedule as she goes in to work earlier than I do.

Anyways, 2000 Jeep Cherokee 4WD 4.0 motor with about 130k on the odometer. Going home about 500 yds from my house the Jeep just started backfiring VERY loudly through the intake/throttle body. Can rev it up and the backfire goes away but it runs rough. Will crank and idle at normal rpm's except for the gunshot backfiring.

One code had popped and it was P0122 TPS, so I checked voltage going to it and it was 5v like book stated, so I replaced the TPS. No luck.

Cleaned TB by spraying cleaner in it while running, keeping it running and not letting it stall out. No luck.

Checked all the wiring, while running move wire harness.

Pulled Cam Position Sensor and smelled burned and since it felt like a timing issue I replaced the Cam Sensor. No luck

Had replaced plugs about 3 months ago with NGK's so those are fine. I pulled coil packs/rail and checked firing. The boots all fire fine.

Checked Fuel pressure at the rail, its reading 49psi at idle.

Checked for vacuum leaks by spraying the TB cleaner all around, no fluctuation in idle or revving.

After replacing both the previous sensors I have disconnected the positive terminal and touched it to the negative for 30 seconds and did the PCM reset steps.

I am at a loss, I have scoured the interwebs because I hate posting problems in the luck that there are others out with the same issues, but again from when I have found similar issues was on earlier XJ's that had distributors, as this one does not I have no luck.
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  #2  
Old May 17th, 2012, 19:27
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Tim_MN Tim_MN is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

I would suspect the O2 sensors. Test the O2 sensors.

O2 sensor service life is about 100-150,000 miles. They become slow to react and can cause bucking, stalling, or backfiring. When they go, you often see an unexplained drop of 30-40% in your gas mpg's.
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Originally Posted by bimmerjeeper

I spent a few hours researching how to jack up an XJ, but even this is more complex than a normal car.
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  #3  
Old May 17th, 2012, 21:26
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sidewaysstarion sidewaysstarion is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Also check the map sensor and the vac line leading up to it.
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  #4  
Old May 17th, 2012, 21:37
stonedv8 stonedv8 is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Awesome, gave me two more good areas to test.

Will do after work tomorrow..

Thanks guys.
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  #5  
Old May 17th, 2012, 22:24
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Pull all the plugs and do a compression test. If one cylinder is way low, then look at it closer. If all 6 are low, then perhaps a timing chain jumped. Sometimes a backfire is an intake valve not fully sealing or closing, and when the plug fires, if the intake is not sealed closed, it will backfire into the intake manifold. A compression test is quick and dirty, and it rules out a bunch of items it compression is all good...
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  #6  
Old May 18th, 2012, 07:36
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Pull the cam sensor drive (you will have to re align it when you reinstall it) and check the teeth on the cam and sensor drive. Before you pull it see how it lines up. Bring #1 up to TDC on the compression stroke, pull the sensor off and see if the hole in the drive rotor lines up with the hole in the drive body. It isn't uncommon for the drive to seize up and shear teeth off.

As stated, also check the MAP sensor and the vacuum line elbow going to the throttle body under it.
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  #7  
Old May 18th, 2012, 07:56
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Always start your troubleshooting with the simple and logical stuff. O2 sensors, and for that matter any of the other sensors are routine maintenance items that you should expect to simply replace when they fail. The O2 Sensors and the Coolant Temperature Sensor are the only ones that will cause backfiring.

After you have checked and tested all the sensors, inspected the wire harnesses and wire plugs for corrosion, damage, and chafed or melted insulation, you might consider some less likely suspects.

Low compression or jumped timing chain are highly unlikely. Even a worn cam sensor drive is not likely, but it is easily inspected.

Do a full re-boot on the ECM.

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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Quote:
Originally Posted by bimmerjeeper

I spent a few hours researching how to jack up an XJ, but even this is more complex than a normal car.
.
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  #8  
Old May 18th, 2012, 08:02
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Quote:
Originally Posted by Tim_MN View Post
The O2 Sensors and the Coolant Temperature Sensor are the only ones that will cause backfiring.
Not true. The cam sensor can definitely cause back firing. Theoretically a failing CPS sensor can as well.
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  #9  
Old May 18th, 2012, 08:31
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sidewaysstarion sidewaysstarion is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Quote:
Originally Posted by Tim_MN View Post

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”

I can't see anywhere in the wiring diagram where the headlight circuit comes anywhere close to the PCM. How the hell would that work?
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  #10  
Old May 18th, 2012, 08:41
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Talyn Talyn is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

The PCM doesn't know if the lights are on or off. I don't think that part matters, unless it has something to do with the PCM sensing voltage.
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  #11  
Old May 18th, 2012, 10:20
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

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Originally Posted by Talyn View Post
The PCM doesn't know if the lights are on or off. I don't think that part matters, unless it has something to do with the PCM sensing voltage.
the only think I could think of would be to do that with the battery disconnected to put a load on the electrical system to drain any remaining voltages from the pcm caps (which touching the B+ cable to the block would do). But with the battery reconnected that would just turn the headlights on and off.
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  #12  
Old May 18th, 2012, 15:05
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Quote:
Originally Posted by sidewaysstarion View Post
I can't see anywhere in the wiring diagram where the headlight circuit comes anywhere close to the PCM. How the hell would that work?
IDK, ask the Engineers at Jeep where the instructions came from.

I have done this and it does clear the PCM Adaptive Memory.
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Quote:
Originally Posted by bimmerjeeper

I spent a few hours researching how to jack up an XJ, but even this is more complex than a normal car.
.
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  #13  
Old May 18th, 2012, 15:14
stonedv8 stonedv8 is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Again excellent advice.

I am at work, about to head home and will be borrowing a compression tester to do a test. Its quick and easy enough.

Also, thanks for the info again on reflashing the PCM. I had been grounding out the positive wire on the ground battery wire, so it seems I wasnt following the correct advice. That I can remedy as well.

So, tonights schedule is a compression test, MAP and vacuum test and O2 sensor test.

Also, from Tim_MN advice he stated the coolant temp sensor could cause backfiring, I did notice a large scale/gunk buildup in my coolant cap, (kinda noticed as I was scratching my head on what else to check the last couple of days, a flush is in order) so I may have junk on that sensor. Strange that could cause backfiring, but theres another test as well.
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  #14  
Old May 18th, 2012, 17:29
stonedv8 stonedv8 is offline
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Ok, compression test was good on all 6.

Pulled MAT sensor, cleaned it as it was pretty fouled.

Pulled MAP and checked vacuum. Looked good on vacuum. Did volt test to MAP 5v (good). Did volt output while idling and at WOT. No output whatsoever, stayed at the 5v in. Did not fluctuate at all. Looks like failure so far.

Found another test for the MAP online that had a 3rd test to test if the MAP was grounding, which showed to connect black test lead to #3 pin on MAP connection then run red test lead to positive side of battery. If it shows 12v then its bad, 0v then it could be good. Well it showed 12v, so looks like its the MAP.

Too late tonight to get one, but I work tomorrow so will send wife out and check back after work.

Fingers crossed.
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Old May 18th, 2012, 18:53
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Re: 2000 Cherokee 4WD 4.0 Backfiring through Intake

Quote:
Originally Posted by stonedv8 View Post
Pulled MAP and checked vacuum. Looked good on vacuum. Did volt test to MAP 5v (good). Did volt output while idling and at WOT. No output whatsoever, stayed at the 5v in. Did not fluctuate at all. Looks like failure so far.
Are you certain you are checking the correct wire?

Looking at the plug side, with the clip up. The left one is 1) sensor ground, the middle, is 2) sensor signal to the PCM, the right one is 3) 5v supply. You should be testing #2, the middle one. Its should read some where around 1.5-2v with the engine running at idle.
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