Hi guys, just signed up to get some help. My parent's 1998 Navigator is running TERRIBLE! The idle is so rough the entire car shakes and the rpms jump up and down, but when you give it gas it runs fine all the way through the powerband. We replaced the o2 sensors and are putting in a new fuel filter in a few days. I didn't know if it could be the IAC or what I have very limited knowledge about the Navigator engine. Any help would be greatly appriciated.

:W to :V I don't kow about the Navigator engine, but my town car was acting the same way, i had the Egr channels cleaned , and the IAC valve cleaned, it is running good now, also check to see if any vacumm lines are bad, hope this helps.

that could be anything from water in the tank , bad gas , to timing , or plugs check all of the above

Thanks for the info. I will try those things and post back if it doesn't help! Hope I've got it narrowed down now.