An exhaust manifold of an internal combustion engine is the very first exhaust component bolted directly to the engine. The manifold then attaches to the front exhaust pipe, which in turn attaches to other pipes, a catalytic converter and, eventually, the muffler and tailpipe of the vehicle.
The exhaust manifold bridges the gap between the engine block and the front exhaust pipe or catalytic converter. Bolted tightly to the engine with a gasket in between the manifold and the engine block, the engine's exhaust dispenses spent fuel and air into the manifold at an extremely high temperature. From there, the exhaust purges its way downstream through the remaining exhaust system and out the tailpipe. The manifold has to be made of an enduring metal such as cast iron to handle the temperature of the engine's exhaust out of the block. In later-model cars that have weight restrictions, it is common to find lighter weight manifolds that can crack more easily under the stress of heating and cooling engines.
Some vehicles may have two manifolds, located on either side of the engine or stacked one in front of the other. The purpose of two manifolds may be to offer dual exhaust or simply to increase the horsepower of an engine. Leaks in a manifold are often cracks in the cast iron due to the intense temperatures of the engine. Cracks may start out as small as a hairline crack, which may be noticeable when the vehicle is first started by the volume of noise it makes near the engine. Once the manifold heats up quickly, the crack might self-seal as the metal of the manifold expands; the noise coming from the crack then becomes less noticeable. However, after time, the hairline crack will begin to increase in size due to the constant heating and cooling of the engine. The manifold gasket is another area of concern that has to sustain the expanding and contracting of the heated metal. Its symptoms may start similar to the hairline crack and could inevitably become a larger problem.
An exhaust leak can be hazardous for several obvious reasons. While an exhaust leak downstream in the exhaust system can still be problematic, one coming directly from the manifold off the block of the engine creates a much higher risk. Carbon monoxide poisoning can occur to the occupants inside the cabin of the vehicle, especially if the windows are closed and the air vents are open, allowing the odourless and colourless gas to enter the cabin.
The manifold is on the lower end of the vehicle's engine. It may be in the front centre, on the side, on both sides (dual manifolds), or front centre and rear centre (dual manifolds). To effectively inspect the manifold, the vehicle must be placed on a lift. Follow the exhaust system from the tailpipe to the engine to determine the location of the manifold. Some later-model vehicles may even have an initial catalytic converter bolted to the manifold and then attached to the front pipe.
A leaking manifold is unsafe and also harmful for the environment. Because it is not filtered through a three-way catalyst or monitored by the oxygen sensors, the exhaust leaks into the atmosphere haphazardly. A cracked or leaking manifold will create less back-pressure, poor performance in the engine and decreased fuel efficiency; it will allow more pollutants to escape into the atmosphere and disperse deadly carbon monoxide. It can also damage other exhaust components downstream in the exhaust system, such as oxygen sensors, and compromise the efficiency of the catalytic converter.