The intake manifold serves as a temporary reservoir and mixing chamber for atomized fuel that comes from the carburetor or throttle body. It also serves as a sink and cooler to keep excess heat from boiling the fuel in the carburetor or throttle body. Some manifolds have a vane construction to direct precise amounts of vaporized fuel into the cylinders while other manifolds have single tubes to direct fuel flow into each individual cylinder.
Plastic Intake Manifolds
Plastic intake manifolds were constructed for several Ford and GM cars with V-6 and V-8 engines. Some of the problems associated with them deal with the angles cut into their mating surfaces on the manifold that do not align with the block angles. Once they have been torqued together, the manifold distorts to fit the block. Gaps and cracks can result, which can cause vacuum leaks, rough idle and misfire. This can also cause water leaks in the manifold coolant passages.
Intake manifolds made of aluminum or cast iron may have casting irregularities or imperfections pertaining to their molecular makeup. The most obvious signs will be cracks along the casting seams where the metal has weakened because of expansion and contraction. Hissing or sucking noises will be heard from the crack or seam split. The engine will run rough, particularly at idle. The carburetor mixture screw will also fail to adjust.
Central Port Injection Intake Manifolds
On General Motors' 4.3L V6 Vortec engine, the intake manifold has a central port injection system. This injection system has a plenum cover hiding it from view. A problem sign with the intake manifold will be fuel pressure regulator leaking raw fuel into the passenger's side of the manifold while the fuel supply and return lines may leak inside the manifold on the driver's side. Additionally, the main single injector and the spray nozzles can leak. This results in too much fuel sent to the cylinders, causing a miss and a very detectable smell of gas in the engine compartment.
Manifold Water Leaks
Intake manifolds can fail at the mating surface with the block or suffer a gasket failure. Intake manifolds that contain internal coolant passages can split at the gasket edge and allow water to leak externally from the manifold into the valve cover area. This depletes the coolant level in the overflow reservoir and can cause the engine to run hot and misfire. A gasket failure can happen when a water jacket breaks and crosses with the intake port to the cylinder, which can short out the spark plug electrode. Both conditions will cause overheating, engine miss and noticeable water vapor exiting the tailpipe (for the internal break).