By the meaning of gassy like natural gas, does it smell like rotten eggs?
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There Is A Rotten Egg Smell Coming From The Exhaust:
Whenever you run the engine and are not moving, you notice an awful smell from the exhaust. The smell is like that of rotten eggs. Not only do you notice this, so does everyone around your car. You may also have noticed that your gas mileage has been worse lately.
Possible causes:
1. There is a problem with your electronic engine control system: Scan injection and emission control system for Diagnostic Trouble codes. Replace or repair as required.
2. The ignition timing may be set wrong: Check and adjust ignition timing.
3. Your fuel pressure regulator is bad: Replace the fuel pressure regulator.
4. The engine may have mechanical problems: Check vacuum lines and fittings for leaks. Check engine compression. Check for broken fittings and mounts.
5. The engine is running too hot: Check cooling system, thermostat and cooling fanes for proper operation