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What Causes an Exhaust Manifold to Crackor Leak
Your car has two manifolds – an intake manifold and an exhaust manifold. Both serve essential purposes, but the one most likely to experience problems over the long term is the exhaust manifold. Depending on your make and model, your manifold might be a single piece of cast iron with channels/ports built into it, or it could be a collection of pipes tied together. The basic job of the exhaust manifold is to take the gases from each cylinder and send them to the exhaust pipe.
Why manifolds crack and leak
As you can imagine, exhaust manifolds are subject to intense heat. They're also subjected to considerable expansion and contraction as they heat up and cool back down. Over time, this leads to metal fatigue (both cast iron and other types of exhaust manifolds are subject to this). As the fatigue becomes worse, it's possible for the manifold to develop cracks.
Another potential problem lies with the exhaust manifold gasket. The gasket sits between the manifold and the engine block, and is designed to seal the small gap that exists between these two components. Like the manifold itself, the gasket is subjected to significant heat, as well as expansion and contraction. It will eventually fail (this is normal, and cause by nothing more than general wear and tear). When it fails, it will begin to leak.
Problems associated with manifold cracks and leaks
There are several problems created with exhaust manifold cracks and leaks. First, hot exhaust gases are now being vented under the hood, rather than being sent downstream through the exhaust pipe. This can damage plastic components in the engine bay. It can also become a health hazard, as the exhaust fumes can enter the car's cabin.
There's also the possibility that it will affect engine performance. If your exhaust manifold is cracked or leaking, the back pressure in the exhaust system will be incorrect, which can reduce engine power, cause sputtering and other problems. Of course, you won't pass emissions testing, either.