A port fuel injected engine has a puff noise in the exhaust when idling. what caused the problem. A a burned exhaust valve B excessive fuel pressure C a restricted fuel line D a sticking fuel pump valve Excessive fuel pressure or a restricted re turn line causes a rich air-fuel ratio. Therefore Band C are incorrect. A sticking fuel pump check valve may cause hard starting, so D is incorrect. A burned exhaust valve causes a "puff" noise in the exhaust