If you have soot coming out from the manifold, you definately have a leak - the gasket is burned through at that spot. Really, the only way to fix it is to replace it. You can get manifold gaskets at any good auto parts store to fit your vehicle.
If you replace it: When loosening up the bolts, it is suggested to tighten them just a hair first to break it free, then unscrew them. Clean the engine where the manifold mounts and clean the manifold where it meets the motor. (Putty knife works great) I would clean all the bolts real well and wipe a little "anti-seize" cream on the bolt threads. Make sure you tighten the manifold bolts in an alternating type pattern to get them tightened little by little, and evenly across the gasket. Find out what torque is suggested on the manifold, and torque them accordingly.
I hope I havn't insulted your intelligence by posting this reply! I just wanted to be thorough in the process of replacing a gasket.
Good luck!
