Welding a cracked exhaust manifold can save you a lot of money. The task can be done with just about any welder meant for steel. The most important part of welding the manifold is the preparation work; a well-prepped welding job goes very smoothly.
Remove the exhaust manifold from the vehicle. Welding the manifold while it is still on the engine is very dangerous because flammable chemicals are everywhere. Depending on the year, make and model of the vehicle, the removal task can take anywhere from ten minutes to an hour or more. Space and accessibility are the two main factors.
Clean the manifold. Using a product such as a calcium, lime and rust remover, thoroughly scour the manifold to ensure that flammable chemicals do not remain. You can use a wire brush or wheel to really get the tough stuff off. Once the manifold is clean, allow it to dry.
Grind the area to be welded. Grinding the cracked area of the manifold removes any debris or rust that may have built up in the surface of the crack, and also provides a clean place for the welder to create an arc. Don't grind too much material away. Just grind until the cracked area of the manifold is clean and ready to weld. For spots that a grinder cannot reach, do your best with a wire brush to ensure contaminants are removed. It is also a good idea to grind a clean spot where the ground clamp will be attached to guarantee full power when welding.
Weld the crack. Because the metal of exhaust manifolds is normally very thick, you can turn your welder's power up almost all the way, especially if the machine runs on 110 volt house current. The key is to penetrate deep enough to really repair the area, instead of simply patching up the surface. Move very slowly when laying the weld to create a big molten puddle. If the welder is a stick unit, most areas of the manifold should be reachable when welding. Wire-feed machines sometimes have trouble getting into the tight spots, but if both ends of the crack can be welded, it will not travel any further. If using flux-core wire, consider taking the nozzle off in order to make the gun smaller for the hard-to-reach areas.
Place the manifold back onto the engine. After the welding is done and the manifold has cooled, bolt it back up to the vehicle according to the manufacturer's recommended torque specifications. Inspect the gasket to make sure it is not damaged, and replace if necessary. Once the manifold is hooked back up to the head and the exhaust system, you may operate the vehicle.