An exhaust manifold is a component of the exhaust system on a vehicle. It is located between the cylinder head of the engine and the rest of the exhaust system and collects exhaust gasses as they exit the engine.
Any internal combustion engine produces exhaust gasses. These gasses are extremely hot - upwards of 1,000 degrees Fahrenheit - and harmful to humans and the environment. They contain unwanted byproducts of combustion, such as carbon monoxide, hydrocarbons, and nitrogen oxides.
The exhaust system directs these gasses away from the vehicle occupants while suppressing the extreme noise caused by the engine. The first component in the exhaust system is the exhaust manifold, which is a large metal chamber or collection of chambers that bolts directly to the cylinder head of the engine where the gasses escape during the exhaust stroke of each cylinder. The manifold collects the exhaust, sends it to the catalytic converter located downstream in the system, through a long pipe to the muffler, and out of the tailpipe.
Replacement of an exhaust manifold on many vehicles, especially those with transverse (sideways) mounted engines (most vehicles with front-wheel drive), can be challenging. One reason is that the bolts that hold the manifold in place, not to mention the other hardware tied to the exhaust system, tend to collect corrosion and are difficult to remove. Another reason the job can be complicated is that there are many other components that need to be removed for access to the exhaust manifold.
In order to replace the manifold, a technician must decide whether it is simpler to remove a number of components and still wrestle with the manifold mounting bolts or remove the entire engine. That decision might come down to the specific vehicle make and model, as well as the preference of the technician.
At the very least, engine covers, exhaust pipes, and heat shields must be removed. Coolant lines, filters, hoses, alternators, AC components, and other parts might also need to be removed. Care must be taken with the mounting bolts to prevent breakage inside the cylinder head, otherwise the engine might need to be partially disassembled in order to repair the damage.