Are you an EV owner who has received a shockingly high quote for repairs? A reporter would like to speak with you; please reach out to [email protected] by Friday, May 26 for more details.

2004 Jeep Grand Cherokee Laredo rough idling after replacing Spark Plugs

kjhiltonkjhilton Member Posts: 3
edited June 2018 in Jeep
I have a 2004 Jeep Grand Cherokee Laredo 4.0 straight 6 that I just replaced the spark plugs in yesterday. After replacing the spark plugs, the jeep idles rough. the RPM isn't bouncing and in park it is around 650 to 700 RPM. But, it is now idling rough. If in neutral, it seems ok and when driving there is no roughness, but when I put it in reverse, or I am at a stop while in drive the jeep starts to shake real bad. I watch the RPM needle and it isn't bouncing or anything, but just feels like it is going to stall, but doesn't.

Any thoughts?

Also, We replaced the TPS about a month ago and the jeep was running fine. Only since changing the spark plugs has this issue come into the picture.

Thanks in advance.


  • kjhiltonkjhilton Member Posts: 3
    Another thing I just noticed, with the AC on it is rougher. If I turn AC off it idles somewhat ok.
  • thecardoc3thecardoc3 Member Posts: 5,630
    The same answer I always give to a situation like this. Forget what was just done, test and prove what is going on right now. Are all of the cylinders getting good spark? (Use a tool like an ST-125 to find out) Are the injectors being triggered? Can you identify which cylinders aren't firing? Try unplugging the injectors one at a time and watch to see how much the engine rpm/roughness changes. That might at least allow you to concentrate on just the ones that are affected.

    Trying to just guess what "might" have gone wrong would have people tell you to check the firing order to make sure that you didn't cross the wires, but that's assuming this is a distributor equipped engine and not a distributor-less ignition system. Did you replace the wires or coil boots with the plugs?
  • kjhiltonkjhilton Member Posts: 3
    Hey thecardoc3,
    Thanks for your reply. On this particular motor, it doesn't have plug wires or distributor. It has the coil style setup. I will test the injectors as suggested.

    Also, the coils/rail looks new by the way. I did pop out the plugs and rechecked to make sure all were gapped as needed (I did this before I installed them too), but also noticed black sut/carbon on the new plugs when I pulled them out. None were wet which is good. I have run Sea Foam spray through the TB and noticed a little difference.

    Anyway, I will start with the injectors and update my findings. Again, thanks for the help.
Sign In or Register to comment.