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post #1 of 12 Old 07-03-2018, 06:24 PM Thread Starter
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Bucking / backfiring through intake...

New problem started happening the other day and I am suspicious of the new MSD ignition coils that were installed about a month or so ago.

The jeep is bucking and backfiring out of the intake. It is somewhat random and does not surface until I've been driving a while through traffic on a fairly warm day. What happens is that I can be hammering on the throttle or just driving briskly down the highway and then when I slow down and then ease into the throttle again, is when it surfaces. The other scenario is similar. I can come off of a freeway and then sit in traffic at a stop light. As soon as I go to accelerate, it'll pop out the intake.

Mine is a 3.6 so it has the coils over the plugs (no distributer or plug wires per se).

It's not throwing any codes, which is typical of some ignition problems. Normally a code is not thrown unless a component shows open circuit or a short to ground or is operating way out of bounds.

Outside of a bad ignition coil, I am also suspicious of a misbehaving crank angle sensor on the side of the block that has not gone bad enough to throw a code.

I'll figure this out but just documenting here to let others know how I delta with this and what I found. I think that it took warmer days and driving in heavy traffic (now that I have a new job) to surface the problem.

First thing I am going to do is swap out the three new ignition coils on the driver's side back to the originals. That's the easier side now that I have the Edelbrock blower on there. The I'll swap the other three if the problem persists.

If anyone else has hit an issue like this on the pentastar, I'd be interested to know what you found. I've had to diagnose a lot of drivability issues in the past and this is really feeling like its ignition related where the coil is firing too early or too late for some reason or double firing. Maybe a cracked boot? But those were new too.
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post #2 of 12 Old 07-07-2018, 07:29 PM Thread Starter
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Just wanted to follow up and possibly close the loop.

The problem ended up getting worse and then finally started throwing codes - P2098 a few times and one time with P0430.

These are codes for bank 2, which is the driver's side on this motor. Checked for intake and exhaust leaks but none.

I am right at 60k miles on the original O2 sensors and have been running a lot of octane boosters with MMTs through the system (up until a few tank-fulls ago). MMTs have been known to contaminate O2 sensors so I went out on a limb and replaced all 4 with new Bosch sensors. If I nailed it, then I think the upper sensor on the driver's side was going wacky when it got very hot.

So far so good and It just runs a lot better all around but I am not giving it a clear bill of health until I run back and forth to work in the traffic.

I'll report back in a few days and post a pic of the orange coating on the sensors.
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post #3 of 12 Old 07-08-2018, 09:43 AM
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Quote:
Originally Posted by White13JKUR View Post
Just wanted to follow up and possibly close the loop.

The problem ended up getting worse and then finally started throwing codes - P2098 a few times and one time with P0430.

These are codes for bank 2, which is the driver's side on this motor. Checked for intake and exhaust leaks but none.

I am right at 60k miles on the original O2 sensors and have been running a lot of octane boosters with MMTs through the system (up until a few tank-fulls ago). MMTs have been known to contaminate O2 sensors so I went out on a limb and replaced all 4 with new Bosch sensors. If I nailed it, then I think the upper sensor on the driver's side was going wacky when it got very hot.

So far so good and It just runs a lot better all around but I am not giving it a clear bill of health until I run back and forth to work in the traffic.

I'll report back in a few days and post a pic of the orange coating on the sensors.
You do want to be careful with fuel additives and be sure they are O2 and CAT compliant. A wonky O2 sensor can wreck things and make you want to pull your hair out at times.


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post #4 of 12 Old 07-08-2018, 11:02 AM Thread Starter
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Quote:
Originally Posted by 0lllllll0 RPMEXTREME View Post
You do want to be careful with fuel additives and be sure they are O2 and CAT compliant. A wonky O2 sensor can wreck things and make you want to pull your hair out at times.
Yeah, it threw me for a loop at first until the codes surfaced. That helped to give me a better starting point for diagnosing it. I have chopped OEM fender so pulling out the liners was a snap and replacing all four sensors was probably even less time consuming than swapping 6 coils.

I knew that I was playing with fire on the octane boost. I had read mixed feedback from folks that claimed to have used it (specifically MMTs) over long periods of time and many miles without issues. I have read that it contaminates sensors and spark plugs too. I've run two cases of the Lucas and one case of VP Racing's Octanium through this motor and set of original sensors. On top of that, I sprayed in excess of 60 gallons of methanol (really, 120 gallons of MW50) through the motor too. I've given the O2 sensors a pretty hard life.

The two sensors in the middle are from the driver's side that threw the code. The one on the left is from the top and the one on the right is from the side of the cat. The out of the top of the cat was the wonky one.



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post #5 of 12 Old 07-09-2018, 04:43 PM Thread Starter
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DAMN! Problem persists. It takes getting the jeep really hot like through traffic on a hot day to surface the problem.

The jeep does seem to run better other wise because of the new sensors.

It's throwing the same codes, P2098 and P0430. The P2098 code is bank 2 too lean and the P0430 code means the catalytic converter is not doing its job. I think the PCM is telling the truth and something is causing it to go lean and misfire on the driver's side cylinders. The engine probably is too lean and there is no work for the converter to do hence the P0430 code.

Hmmm... gotta leave town in a few days so need to get this figured out.

Going to rule out the following at least for now....

...ignition coils / spark plugs because I think this would result in unburned fuel so a rich reading versus lean.

...bad gas or dirty fuel filter because I think that would be more of a global problem affecting both banks.

...intake leaks because that would be a more constant problem versus intermittent and on a warm day after a lot of driving.

I'm going to poke around for a loose ground on the driver's side of the engine or maybe a loose injector wire.

If anyone else has hit this issue and has any clues, please chime in.
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post #6 of 12 Old 07-09-2018, 05:33 PM
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Likely you have already considered it but possible exhaust leak? The comment on high temp exposing the issue would lead me to check the integral manifold flange for a leak.
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post #7 of 12 Old 07-09-2018, 06:23 PM
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Exhaust leak and engine mechanical issue are common to the two codes you have.

Following is some info for the P2098 code:

P2098

Set Condition: The conditions that cause this diagnostic to fail is when the upstream O2 Sensor becomes biased from an exhaust leak, O2 Sensor contamination, or some other extreme operating condition. The downstream O2 Sensor is considered to be protected from extreme environments by the catalyst. The Powertrain Control Module (PCM) monitors the downstream O2 Sensor feedback control, called downstream fuel trim, to detect any shift in the upstream O2 Sensor target voltage from nominal target voltage. The value of the downstream fuel trim is compared with the lean thresholds. Every time the value exceeds the calibrated threshold, a fail timer is incremented and mass flow through the exhaust is accumulated. If the fail timer and accumulated mass flow exceed the fail thresholds, the test fails and the diagnostic stops running for that trip. If the test fails on consecutive trips, a DTC is set.

2008 Unlimited Rubicon Automatic
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post #8 of 12 Old 07-09-2018, 06:53 PM Thread Starter
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Quote:
Originally Posted by cjindna View Post
Likely you have already considered it but possible exhaust leak? The comment on high temp exposing the issue would lead me to check the integral manifold flange for a leak.
Quote:
Originally Posted by ronjenx View Post
Exhaust leak and engine mechanical issue are common to the two codes you have.

Following is some info for the P2098 code:

P2098

Set Condition: The conditions that cause this diagnostic to fail is when the upstream O2 Sensor becomes biased from an exhaust leak, O2 Sensor contamination, or some other extreme operating condition. The downstream O2 Sensor is considered to be protected from extreme environments by the catalyst. The Powertrain Control Module (PCM) monitors the downstream O2 Sensor feedback control, called downstream fuel trim, to detect any shift in the upstream O2 Sensor target voltage from nominal target voltage. The value of the downstream fuel trim is compared with the lean thresholds. Every time the value exceeds the calibrated threshold, a fail timer is incremented and mass flow through the exhaust is accumulated. If the fail timer and accumulated mass flow exceed the fail thresholds, the test fails and the diagnostic stops running for that trip. If the test fails on consecutive trips, a DTC is set.

Gents,

Thanks for the feedback. It is VERY much appreciated since getting this diagnosed, parts located and then repair performed means that I go to the Rubicon this weekend.

I double checked for exhaust leaks. With an exhaust leak, as you probably know, there will be soot build up around the leak especially pre-cat. Anyway, I'm an old school tech and I have a mechanic's stethoscope. You pull off the mechanical diaphragm / amplifier thingy with the metal rod and just use the tubes. Of course I did that too around both the intake and exhaust manifolds. I can see an exhaust leak causing the codes but do not see an exhaust leak causing the drivability issue that I am observing when the codes get triggered.

Also, I think an exhaust leak issue would get progressively worse and keep reoccurring as the motor got hotter and more heat saturated.

What I am observing is what I would liken to a float bowl running dry coming out of a corner on a carbureted vehicle where it sputters badly and then clears or an ignition problem where it stumbles on initial acceleration and then clears and drives normally again until it reoccurs.

Maybe I will just swap the coil packs for peace of mind. I could swap bank one with bank two to see if the problem follows the coils. Never mind putting back the old originals back in.

The other thing I was thinking was something fuel line related or just fuel related. The fuel line feeds the passenger side bank first and then loops around the back of the lower supercharger manifolds to the drivers side rail. thinking a couple of things here. Maybe overheating the fuel in the rail or not enough pressure getting to the passenger side. If the fuel pump relay in my TIPM is going out (worn out contacts?) then maybe I'm loosing fuel pressure at the end of the line? Maybe a plugged fuel injector.

Last edited by White13JKUR; 07-09-2018 at 06:56 PM.
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post #9 of 12 Old 07-11-2018, 03:21 PM Thread Starter
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It sounds like I was on the right track with a possible crank position sensor. It finally failed hard today and threw a P0339 "Crankshaft Position Sensor A Circuit Intermittent."

Locating the parts now. I hope this nails it and I can make the Rubicon this weekend. Still time. FWIW, I keep one of those TIPM fuse bypass thingies in the glove box in case I lose the fuel pump relay on the TIPM on the trail.
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post #10 of 12 Old 07-11-2018, 05:25 PM Thread Starter
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Okay, drove through 100 degree weather with jeep falling on it's face intermittently, all the way to the dealer. I cleared the codes before the dive and it threw all three above mentioned codes again.

Now when it fails, it fails badly on the highway to the point that I have to hit the e-flashers and move over to the side of the road. I let it cool for a minute and then drive away like it never happened.

The crank position sensor is a pretty easy fix. I could have probably done it in the jeep dealer's parking lot in 5 minutes with a 10mm wrench but drove back home through the heat.

I can see why that thing failed and why my observations are tied to getting the jeep heet saturated on a warm day. It is awfully close to the passenger side cat and the plastic was hard as a rock. Had to be careful not to break the connector.
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post #11 of 12 Old 07-12-2018, 08:57 AM
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I had something similar happen right after installation of my SC, same codes too. Replacing O2 sensors seemed to help, but didnít. Solution was replacing the spark plugs with a narrow gap NGK and itís been great since. Hope you get it sorted out.
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post #12 of 12 Old 07-18-2018, 10:38 AM Thread Starter
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Originally Posted by Asinine View Post
I had something similar happen right after installation of my SC, same codes too. Replacing O2 sensors seemed to help, but didnít. Solution was replacing the spark plugs with a narrow gap NGK and itís been great since. Hope you get it sorted out.
Well, I now see that I had two separate problems.

The post-heat-soak bucking that I was experiencing and the P0339 code were definitly due to a bad crank position sensor. That thing not only pokes into the crank case and comes into contact with hot engine oil, it is between the catalytic converter and the block on the passenger side so that little sensor sees some heat. I replaced the sensor last Thursday and then drove to the Rubicon, through it and back in the heat and not a single issue.

In fact the sensor has been causing me grief for a long time, i can now verify. I've been getting very mild hesitations in the mid range and near redline. I was also getting an occasional cut out when accelerating hard under load where it would buck hard and then keep climbing the RPMs. Never any coded and now all those issues are gone.

I would also say that it runs a little better at super low RPMs out on the rocks. I have a much better stall point now than I did before. I noticed it immediately on the Rubicon because I could now run into a lower RPM when crawling before stalling the motor. It works a LOT better now at the super low RPMs. I had to relearn my stallpoint at first. I still manage to stall the motor from time to time though with my manual transmission.

Replacing the o2 sensors also gave me something. It runs a lot smoother from that as well. Even through there were no identifiable leaks at the sensors, I am getting a lot less popping and crackling in the exhaust on decel.

It actually runs better than ever now - very responsive and does not skip a beat from idle to redline even through it is still throwing the P2098 and P0430 codes. I hit those codes this weekend on the Rubicon but without issue. Cleared them and hit them again on the freeway drive home. Again, it was running perfectly and the light just came on.

Ignition coils are new and the plugs are not super old.

I'll keep poking around until I get P2098 and P0430 nailed. Runs perfectly otherwise.
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