TongMan

The problems associated with exhaust leaks vary by the size of the leak, its location and what type of fuel injection system your car has. Although most people think that exhaust leaks are little more than a source of irritating noise, the truth is that they can affect performance, fuel economy, emissions and present a danger to the car's occupants.

Noise
The level of noise associated with an exhaust leak varies with its location. In general, the noise emitted by a leaking exhaust will get louder and sharper with proximity to the engine. A leaking muffler will probably only produce a low grumble, a leaking mid-pipe in the catalytic converter will produce a loud drone at highway speeds, and an exhaust manifold leak will emit a pronounced rap that rises with RPM.

Carbon Monoxide Poisoning
The single most dangerous aspect of an exhaust leak is carbon monoxide poisoning. This colorless and odorless gas is a byproduct of combustion and, when inhaled, replaces oxygen-carrying hemoglobin in the blood. The immediate effects of carbon monoxide poisoning are nausea, headache and vomiting, followed by lethargy, unconsciousness and ultimately death. The EPA has concluded that one out of five vehicles involved in an accident may have elevated levels of carbon monoxide in the cabin, which ultimately caused the driver to become inattentive. Carbon monoxide poisoning can be recognized by a flushed complexion, and blood that is a bright red (almost orange) in color.

Performance
If your car was produced after 1995 and has an exhaust leak upstream from the catalytic converter, then the exhaust leak will almost certainly cause a reduction in performance. This can occur because the engine's oxygen sensors are not receiving correct information about the engine's air-fuel ratio, which will cause the engine to inject more or less fuel to compensate. This decreases engine performance, particularly in the low-RPM band.

Fuel Economy
The performance decrease caused by an overly-rich fuel condition will require the driver to use a heavier foot while driving, which will inevitably lead to a decrease in fuel economy.

Catalytic Converter Meltdown
Catalytic converters use a matrix of metal to transform un-burned fuel into less harmful compounds but can only cope with so much. Too much fuel in the exhaust stream will cause the converter's internal temperatures to rise over 500 degrees, melting and cracking its internal matrix. This condition is called "converter melt-down" and can be extremely dangerous. An overheated converter can easily flash-ignite any surrounding combustibles, such as oil, transmission fluid and grease.