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1995 ZJ 4.0, rough idle

Old 01-05-2019, 02:51 PM
  #1  
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Year: 1995
Model: Grand Cherokee (ZJ)
Engine: 4.0
Default 1995 ZJ 4.0, rough idle

I have a 1995 ZJ with the inline 6 4.0L engine. Recently it has been idling rough, especially when in gear at a traffic light for example. In the last couple of months I have also had trouble with the overdrive not turning on when I initially start the car (the "overdrive off" button is lit when this happens). Not sure if both issues are related. I have already replaced the throttle position sensor and idle air control sensor. The throttle position sensor was replaced a few months ago, after the overdrive was acting up and stopped engaging all together. Replacing the TPS fixed that all together. But now the car acts like it wants to die when I stop at a traffic light. As soon as I give it gas, it drives just like normal. Any advice would be greatly appreciated!

Last edited by CJ Lopez; 01-05-2019 at 02:56 PM.
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Old 01-06-2019, 04:31 PM
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Year: 1995
Model: Grand Cherokee (ZJ)
Engine: 4.0
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I forgot to mention before, I also just replaced the Crankshaft position sensor last week with no change in how it idles. Today it has started to feel a little under-powered, and it stalled while idling at a stop sign. I am no mechanic so any advice or ideas of what to check into is appreciated!
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Old 01-09-2019, 03:02 PM
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Model: Grand Cherokee (ZJ)
Engine: 4.0
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I have decided to track and post my progress with this current issue in case it can be a help to someone in the future. This week, we pulled out all the stops and looked at every little possibility. We started by replacing the MAP sensor, which did slightly improve the rough idle condition, but did not solve it all together by any means. Next we replaced the fuel filter after realizing how old it was. After checking the inside, we were glad we did. Again, no improvement to the rough idle. Next we went to the cap, rotor, spark plugs and wires. Upon replacing these, we found that the clip in the #6 spark plug wire was disconnected from the rest of the wire and was latched on the spark plug. We also found the #3 spark plug to be loose and not all the way fastened. At first we celebrated in victory as the Jeep seemed to be handling near perfect with new plugs, wires, etc. But this morning on my way to work, the Jeep started dropping RPMS again, not only when stopped in gear but also when driving and under load. Still no check engine lights or codes of any kind besides a bad coolant sensor which we have known about since buying the Jeep. We believe it to be the fuel pump, and are guessing that it has been the primary cause of this issue from the beginning, though lots of what we have done was much needed. Hopefully the fuel pump is the last of it, and I will have a freshly tuned up Jeep by the end of this week! Again, any comments, questions, advice, etc. are greatly appreciated,
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