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Old 08-14-2010, 08:55 PM   #1
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New Jeeper and misfire

Bought my first jeep this week. 97 Sahara 4.0L. Beyond excited about joining the club

Getting a misfire code. Individually unplugged and replugged the injectors with a noticeable difference in all except with the first cylinder, which makes me think that's the misfiring cylinder.

Replaced spark plugs, spark plug wires, distributor cap and rotator but still getting the code. Thinking it might be the injector but looking for any other suggestions.

Thanks everyone!

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Old 08-15-2010, 05:52 AM   #2
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Good, you narrowed it down to Cyl #1.

Now I am working off an 03-06 4.0 engine control here, so your 97 may be different.

The ECM controls the ground leg for the injectors.

Key on, engine off, using a volt meter, first figure out which line is positive, and ground. Once you find the ground, set the volt meter to ohms and with one lead to the injector harness ground line, and the other to either battery negative or a good ground on the engine, rotate the engine around by hand and see if the ohm reading changes (should be OL or no continuity 99% of the time while you are rotating it, then a very brief moment of continuity). If you are getting a low ohm reading constantly, then your injector control wire is shorted to ground. Likewise if you can not find the positive wire - shorted or open (broken). If that is all testing good, then, try swapping the injector to another cylinder and see if the problem moves.

Once you isolated which is the problem, then start checking the wire harness for pinches or rub through. WCS it is an ECM problem. It could also be a dead cylinder (low compression).

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Old 08-15-2010, 10:14 AM   #3
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Some '97-98 TJ 4.0L engines came with a bad (weak) set of valve springs which will cause a VERY difficult to diagnose misfire on one or more cylinders. My '97 had that problem. Jeep has a TSB on the problem which basically replaces the bad valve springs which should cure it if that is the issue. The only thing is that if that problem has gone on long enough, it could have burned the valves or valve seats which would require a valve job too to completely correct the problem.
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