Exhaust leaks of any kind are annoying and dangerous, but cracked manifolds present an entirely new level of danger. These cracks usually occur at the manifold flange and are the culmination of age, manufacturer defects and repeated heat cycling. Cast-iron manifolds can often be repaired, and tubular manifolds can be replaced, but either way, this is one problem you can't afford to ignore.
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Air Fuel Ratio
If the crack siphons in cold outside air into your exhaust system, the oxygen sensor may read this as a lean condition. Consequently, it will inject extra fuel to compensate, resulting in a rich or "choke" condition. This will cause rough idling, poor fuel economy and may result in engine damage under some conditions.
Your car's catalytic converter stores the heat from the engine's exhaust gas and uses it to convert harmful chemical emissions and unburnt fuel into more inert forms. If the computer is allowed to inject excess amounts of fuel, the catalytic converter will catch the hydrocarbons, sending its temperature through the roof. Though these devices typically operate at more than 426 degrees C, the 1500+ degree temperatures encountered under these conditions will rapidly melt its internal matrix and render it permanently damaged.
Not only can a converter meltdown be costly, it is also quite dangerous. If external temperatures exceed 482 degrees C, the converter stands a real chance of flash-igniting surrounding combustibles like oil and grease.
A cracked exhaust manifold will almost certainly leak some quantity of exhaust gas into the engine compartment. These untreated exhaust gases contain large quantities of carbon monoxide, a clear and odourless cousin of CO2. This molecule can replace oxygen in the bloodstream, causing drowsiness, suffocation and ultimately death. The likelihood of this drastically increases in traffic, where the carbon monoxide gases can collect under the bonnet and be drawn into the passenger compartment by the A/C ducts.
The most terrifying possible effect of an exhaust manifold crack is that the front end of your car blows to pieces like a giant fuel/air bomb. Some of the extra fuel injected by your engine's control computer will work its way out of the crack in the form of gas vapour. This hot, raw vapour can collect under the bonnet, filling it to the point of saturation and possibly exploding. Granted, this is not likely to occur unless you sit in traffic for 2 hours without engaging the cooling fan, but it a known possibility.
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