Rough idle problem - vibrates the plastic dashboard on my 2003 infiniti FX35 with Nissan VQ35DE 6 cylinder engine. 220,000 miles. Changed engine mounts, replaced intake/plenum gasket, ran Lucas additive through the tank, cleaned and adjusted throttle body and set idle at 650 RPM (holds steady). Replaced plugs. Still idles roughly. Any thoughts?
those are notorious for blown head gaskets at that age. Do this video test first:https://www.youtube.com/watch?v=QA7KVQq9vKA&t=42s and if not that, realize the coils often go bad on those