Okay, lets start with the exhaust manifold. They need to be torques using a torque wrench. Most part stores will lend or rent one to you. The bolts get torqued to 18 ft/lbs. Start in the center of the manifold and work your way out going left to right.
As far as the idle, it could be a vacuum leak or a bad idle air control valve (IAC).
Yes, I have heard of the rear main sealer and yes often times they work. Just make sure to follow the manufacturer's directions and don't overfill the engine oil.
Let me know how things go for you.
Thursday, June 18th, 2015 AT 7:10 PM