2004 grand cherokee larado 4.0 i6 4x2 rough Idol
 

Got this jeep 3 weeks ago, never owned one, its it excellent condition in and out - 78k miles - The idol seems to be a little rough, not terrible. I changed the plugs, it had Bosch +4 platinum in it, they didnt look to bad but replaced with the same, no difference. I read that these 4.0 dont like Bosch +4 - I cleaned the throttle body and idol sensor, that made a bit of difference. SO I decided to put in champion copper like many have recommended. The idol was worse and had a miss. I doubt its a plug issue, maybe the coil? sensors? If anyone has any suggestions as to what I can try to troubleshoot it or replace let me know, this engine should be purring with the champions in it.

What did the spark plug insulators look like? If they are dried out or cracked, carbon trailed or the contact springs are not good, you could have a miss.

I just bought a Denso set from rockauto.com for $18.32 shipping included. They are not available from Mopar unless you buy the coil pack, $279!

The plugs were new. I replaced the plugs that were in it with the same ones at 1st, the bosch +4, then I tried the champion copper because many said the 4.0 like copper and not the bosch +4's The bosch have a minor rough idle but the new champions had a worse idle and what appeared to me as a miss so I put the new bosch back in.

I'm talking about the rubber boots under the coil pack.