The intake manifold on an engine is a large housing, a series of passageways, that distributes and directs air (and sometimes fuel) into the cylinders inside the engine. An intake manifold gasket seals between the manifold and the engine’s cylinder head.
Attached to one end of the intake manifold on a modern engine is the throttle body that opens and closes to control the amount of air pulled into the engine on its intake stroke. At the other end of the manifold at the place where it bolts to the engine is the cylinder head. When your engine is running, air is drawn through the throttle body by a vacuum (produced by the engine), through the intake manifold tubes or “runners”, and into the intake ports of the cylinder head so that it can mix with the fuel entering the combustion chamber.
Intake manifolds come with an array of vehicle-specific designs. Their main purpose is to carry air from the throttle body to the cylinder head and combustion chambers. But the manner in which it does its job can differ. For instance, on some vehicles, the air will be vacuumed through the manifold. On others, the air is pressurized. Some engines even cool the air with engine coolant that passes through chambers inside the intake manifold.
If your intake manifold is cracked or leaking, not only will the air to fuel ratio be upset, but coolant and oil could also leak. If the manifold is compromised, you might notice one or more of the following signs: