Neon Dohc and Sohc engines have whats called a EGR system, or Exhaust Gas Recirculation system installed from the factory. What the EGR system does, is recirculates spent exhaust gases from the exhaust side of the head, and sends it to the intake manifold. This is done for emissions reasons, and it serves its purpose well.
HOWEVER when you take exhaust fumes, which are rich with carbon soot and put it back into the motor, the soot makes the intake manifold, and throttle body gummed up with a sticky tar like substance. Eventually that sticky tar can build up inside the motor and potentially cause sticking throttle bodies, and cause other problems.