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I have a 1997 Sahara with the 4.0 liter straight six w/ automatic trans. I have been having an issue where the engine will backfire after it is warmed up and running fine. This only happens under load around town at lower RPM settings. If and when the backfiring starts to happen usually I can give it more gas and the trans will kick down a gear then when engine RPMs are higher it goes away. The other way it stops is if I shift into neutral thenthe engine runs perfect again. I have had the trans out and rebuilt then reinstalled by a good trans shop in town. I have changed plugs, plug wires cap and rotor as well as the crank position switch. NO luck so far.


Any help would be greatly appreciated.
 

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Replace the upstream O2 sensor, make sure to only use NTK. Do NOT use Bosch. No need to replace the downstream O2 senso mounted on the catalytic converter.
 

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On my 97 4.0, when I get a backfire under load it is almost always either a bad spark plug wire or the internal resistor in a spark plug that has failed.

Get out your volt/ohm meter and check your wires, then after you take the spark plugs out measure between the top connector and the electrode. Wires over 8000 ohms are not good, a wire that causes a back fire is usually very high to open, same with spark plugs.
 
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