Engines lopes and surges at start up (idle). OK while driving, but hesitates when excellerating from a complete stop. I also smell the order of methan gas inside the cab from time to time, especially noticable at start up. Also I notice a clicking sound at start up or when just idling. It sounds like it is coming from the left front side of the engine. Could these issues be atributed to a bad catalytic converter, vacuum leak in the exhaust. Car does have a loud idle like there is a leak in the muffler. I replaced the COPs about 8 months ago.