The air intake manifold is a plumbing system for engine air. Bolted to the cylinder heads, it receives air from the throttle body and splits it into separate runner tubes feeding each cylinder. The design of these runners is critical; their length and shape are tuned to optimize air velocity and cylinder filling at different engine speeds, improving torque.
Modern manifolds are often made of composite plastic to save weight and reduce heat transfer from the engine block. However, they are prone to cracking or leaking at the mounting gaskets. An intake manifold leak lets unregulated air enter the cylinders behind the MAF sensor, causing lean misfire codes, rough idling, and poor engine performance.