In modern automobiles, the exhaust manifold is part of the larger manifold system. Its function is to remove engine exhaust--air and vapour, along with any uncombusted fuels--and inject it into the exhaust pipe to be carried out and away from the engine. A cracked or otherwise faulty exhaust manifold can result in inefficient fuel consumption and a pronounced drop in engine performance. It can allow harmful exhaust fumes to accumulate in the engine compartment, where they can then make their way into the cabin of the vehicle.
Pronounced Exhaust Odors
The most obvious sign of an exhaust system problem is the presence of exhaust odours in the passenger cabin, though this can also indicate a breach of the exhaust pipe beneath the car. To differentiate between the two, run the engine until it's warm and open the bonnet. If there is a distinct smell of exhaust originating from the engine compartment, the problem is very likely in the manifold.
Performance and Noise Issues
Because combustion engines use the natural vacuum pressure caused by the compression, combustion and evacuation of air and fuel, another symptom of a cracked manifold is an obvious drop in engine performance. Weak acceleration accompanied by a stuttering effect may indicate a manifold problem, but it does not necessarily mean the manifold is cracked.
This obvious loss of performance accompanied by louder engine noise points more definitively toward a breach in the exhaust manifold. The noise will typically manifest as a louder exhaust rumble, or a wide-open sound. Again, a breach in the exhaust pipe may also cause this, so be sure to check under the hood while the engine is being accelerated to ensure that the noise is coming from the engine compartment rather than beneath the car.
Identifying Cracks in an Exhaust Manifold
A cracked exhaust manifold is not always immediately apparent. Stress fractures caused by excessive heat can start small and widen over time, making the effect difficult to notice until the engine's performance is noticeably diminished.
Because most fractures are caused by heat stress, cracks will form where the heat is most concentrated, in this case where the manifold joins the engine. In older manifolds made of cast iron, cracks are usually harder to see, as rust accumulates rather quickly. In newer engines with aluminium manifolds, cracks may be more readily apparent thanks to aluminum's lighter colour and resistance to oxidation. Removal of the exhaust manifold and a close physical inspection may be necessary to locate cracks.