Unless your car has headers, it probably has a manifold through which exhaust gases exit the engine. When exhaust gases are produced by the engine during combustion, they are pushed out the exhaust manifold. Most exhaust manifolds are made of iron or steel, allowing them to withstand hot temperatures -- sometimes reaching as high as 1,200 degrees Fahrenheit. But like most automotive components, exhaust manifolds are susceptible to failure. Over time, manifolds may develop hairline cracks. While a small fracture in your car's exhaust manifold may sound harmless, though, it can lead to some other problems if left unchecked.
Poor Gas Mileage
You may discover that your car gets poor gas mileage with a cracked exhaust manifold. This is because some cars have an oxygen (O2) sensor after the exhaust manifold. Therefore, a cracked exhaust manifold may cause incorrect readings of the O2 sensor, prompting the engine to burn an excessive amount of air (running lean).
Exhaust Gases in Cabin
With a cracked exhaust manifold, some of your engine's exhaust gases will escape into the engine bay rather than being dispersed out the tailpipe. Why is this a problem? Well, your car's climate control system pulls air from the engine bay. If the engine bay is filled with exhaust gases, some of those gases will end up inside in your car's cabin, thereby exposing you and your passengers to potentially toxic fumes.
Damage to Surrounding Parts
In some cases, a cracked exhaust manifold can cause damage to other parts in the engine bay. It's no secret that exhaust gases are hot, and when those hot gases leak through a cracked manifold, they can melt wires or even ignite parts made of combustible materials.
Failed Emissions Test
If you live in a state or city that requires emissions testing, a cracked exhaust manifold may cause your car to fail. It's not uncommon for a cracked manifold to trigger a Check Engine Light (CEL). When this light is activated, your car will automatically fail.
Unfortunately, there's no quick-fix for a cracked exhaust manifold. Some drivers have attempted to use heat-resistant epoxy, such as JB Weld. While JB Weld is an amazing product with hundreds of uses, it simply can't withstand the 1,200-degree exhaust gases. The only viable solution is to replace the cracked exhaust manifold with a new manifold. Once the new manifold is installed, these problems should go away.