Severe Misfire


Tx99tj

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Sep 13, 2020
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Fort worth
99 TJ - this all started last week. Driving down the highway I noticed a bad miss and my check engine light flashed. This continued intermittently until I stopped although the miss and check engine light flash did decrease significantly as I had less of a load on the motor. I drove home and took it easy, stayed off-highway, and it happened a few times more. Over the weekend I changed plugs, rotor, cap, and coil. Still happening. Particularly under more of a load and more frequently as the Jeep is up to temp. I am hoping its not a head gasket. No apparent water in oil or oil in water. Any guidance?
 

Jerry Bransford

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seems to run perfectly
That makes me think you have a bad upstream O2 sensor. If you replace it, install only an NTK or NGK which is the OEM brand. Whatever you do don't install a Bosch O2 sensor which is a brand that for some mysterious reason does not work well in our Jeeps. No need to install the downstream O2 sensor too, it only monitors the catalytic converter and does not affect how the engine runs.
 

Steel City 06

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When this happened to me, it turned out to be my catalytic converter. One of the pre-cats self-gutted and plugged the 3rd cat, resulting in misfires on all cylinders, especially at high RPM and high throttle.

I would recommend unbolting the catalytic converter assembly and taking a peek at all of the cats. If any are plugged, partially melted, or even hollow, they will have to be replaced.
 
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Jerry Bransford

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I did not. How do I reset the computer?
Disconnect the negative battery connector from the post and leave it disco'd for 3-4 minutes. A more thorough computer clear is done by removing both the - and + battery leads from the battery and holding them together for a minute or two. Some carry that even a little further and pull the headlight switch on too to further drain the computer's capacitors that hold programming alive.

You'll have to reset the clock and your radio stations but this will clear the adaptive memory inside the PCM. The adaptive memory is what learns your TJ's operating parameters and stores them.

Doing this will definitely give the new O2 sensors the best chance at helping.

There is definitely no guarantee this will work but clearing/resetting the computer after replacing a critical sensor like the O2 sensor is a good idea.
 
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Tx99tj

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Sep 13, 2020
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Fort worth
Disconnect the negative battery connector from the post and leave it disco'd for 3-4 minutes. A more thorough computer clear is done by removing both the - and + battery leads from the battery and holding them together for a minute or two. Some carry that even a little further and pull the headlight switch on too to further drain the computer's capacitors that hold programming alive.

You'll have to reset the clock and your radio stations but this will clear the adaptive memory inside the PCM. The adaptive memory is what learns your TJ's operating parameters and stores them.

Doing this will definitely give the new O2 sensors the best chance at helping.

There is definitely no guarantee this will work but clearing/resetting the computer after replacing a critical sensor like the O2 sensor is a good idea.

Thanks!
 

Apparition

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When this happened to me, it turned out to be my catalytic converter. One of the pre-cats self-gutted and plugged the 3rd cat, resulting in misfires on all cylinders, especially at high RPM and high throttle.

I would recommend unbolting the catalytic converter assembly and taking a peek at all of the cats. If any are plugged, partially melted, or even hollow, they will have to be replaced.
Same, couldn’t go over 55 or I’d throw a misfire code due to the engine load.
 
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Tx99tj

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Fort worth
As soon as the load got too high it’d throw the misfire code and essentially go into limp mode until I stopped and restarted.
Interesting. Mine flashes, RMP, and MPH gauges bounce around and it goes back to normal or just repeats a few times as it misses.
 
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Tx99tj

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So I have reset the computer. Still the same issue. I think I am going to replace the cat. I am debating on replacing the manifold too. Mine has a leak. May as well while I'm at it.
 

Steel City 06

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My CEL would flash during misfires, and then stay on constant if there had been multiple incidents. Resetting the computer didn't get it out of limp mode.