what causes exhaust fumes smell when you're driving a 2002 pontiac grand prix

Could be an exhaust leak. Or exhaust from another car coming into your car.

If your windows are up and you smell car exhaust and your heater is off that indicates an exhaust leak coming from your exhaust system. There could be a hole or a gasket that has failed allow exhaust gases to be released where they should not be. Patched or replaced is the next step.