Exhaust with an especially strong smell of gas is indicative of a wide variety of possible problems. This is usually called "running rich," as opposed to "running lean," and the gas smell can usually be noticed around the car during idle.
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The air-to-fuel ratio could be off balance -- excess, unburned fuel is exhausted in the form of fumes. This is usually a symptom of larger, more complicated problems, and is what generates the "gassy" smell from exhaust.
Commonly referred to as "O2 sensors," these can fail and cause the car's computer to improperly adjust the air-to-fuel ratio, and then cause the engine to run rich. Usually the car will display a warning on the dashboard, sometimes just as a check engine light.
Catalytic converters work to expunge exhaust gas from harmful byproducts of burnt fuel. Because of this, they can sometimes become clogged, backed-up or otherwise rendered ineffective. This will throw off the air-to-fuel ratio, causing the car to run rich.
All modern cars use some sort of computer management to regulate the engine. Sometimes problems can occur with the software that runs on the car's computer, causing a variety of problems. A visit to a certified mechanic is usually required to examine the computer's status.
Timing Belt or Chain
Timing belts and chains can become stretched out over the life of the engine. It's usually recommended by most manufacturers to change them every 50,000 to 75,000 miles. When they stretch, the timing of the engine's camshafts is misaligned with the pistons, leading to either unspent fuel or not enough air.