It is the GMC Exhaust Manifold that safely transports the dangerous gases from your engine compartment away so the driver does not have to breathe them. It is the exhaust manifolds that are attached to the engine block covering the exit portal from the combustion chamber. Because of this position on your GMC truck, this engine component is exposed to extreme temperatures that can cause them to crack over time.
On most older GMC trucks, the exhaust manifolds are covered with a thin film of rust. This is common since the paint wears off due to the high temperatures this engine component is exposed too. This condition is not something the owner of the truck should be concerned about. This layer is very thin and does not affect the exhaust manifolds ability to perform its duty of safely carrying away the dangerous gases.
The leading reason for the GMC exhaust manifolds to be in need of replacement is when they crack. This cracking generally occurs when the hot manifolds are exposed to extremely cold water when the engine is running. It is the temperature variations of the two conditions that cause the cracks to form which in turn will leak the dangerous gases into the engine and passenger compartment.
The first thing generally noticed when an exhaust manifold cracks is not the gases leaking, but the sound of the engine ignition cycle being exposed. This is extremely loud and not only annoying but will damage the valves in the GMC truck engine if left in this condition for any period of time. The crack reduces the back pressure on the exhaust valve causing it to warp damaging it and reducing its ability to seal the combustion chamber when the valve is suppose to be closed.