Is my exhaust manifold leak over? please help

[B16a2 Sleeper]

Today I reinstalled my factory exhaust manifold, I noticed after driving it about 10 minutes and popping the hood, I saw a lil black smoke coming out from the exhaust manifold area.. I shut off my car and waited for about a hour and drove around the whole day and there was no smoke.My mechanic said its normal that it burns off around the exhaust manifold wen u reinstall a exhaust manfiold. Is it really normal? Oh yeah the exhaust manifold was lying on the floor of a autoshop wen I was reinstalling it. Maybe oil and grease got on it? Please help.

[Milano97]

When I installed my DC Header and started the car, all this white steam looking smoke came out, it smelled nasty...I once put back my stock manifold and there was no smoke, but then again I keep everything well packaged, so yes it might be some oil or residue that leaked into the manifold...Oh...do your headers make a hissing sound? Cause my DC Headers make a hissing sound, kinda like an intake, under throttle. I put back my old stock header and the hissing sound is gone...Everyone said to change the gasket and the donut gasket. I used the gasket that bolts to the block, it was fine, and I used a Mugen gasket for the one under the car that connects to the cat-converter, but I can't tell where the noise is...any suggestions?