2000 Mitsubishi Eclipse 6 cyl Front Wheel Drive Manual 164000 miles
one day the sound of my exhuast dramatically changed from troaty to something more like a chain saw. Engine seemd to be firing fine, but 30 miles later service engine light came on. Took car to local shop. Got multiple misfire code. New plugs, wires, roter etc installed and car tuned. When I picked up the car, light was out but exhaust still sounded funny. Service engine light came on next day. Took car to an official Mitsu dealer, explained problem and change in exhaust sound. They couldn't find anything but suggested we reflash the computer - which I had them do. I picked up the car. Same strange exhaust sound but light was off. By the time I got the car home - service engine light was on again. I am wondering of the problem isn't tied to the catalytic converter or something in the tuned exhaust system that has failed and caused the change in exhaust sound and maybe adding backpressure that might be affecting the exhaust and tripping the light. So.
I just had Autozone recheck the engine code and it was P0300 multiple Cylinder misfire which it has been the last two times I've had it in for repairs over the last week. Keeping in mind that I just had new plugs, wires, cap, rotor and tune up one week ago, then 2 days ago the Mitsu dealer checked it, reflashed the computer, light went off for 24 hours but came back on. All the while my exhaust still sounds different. The dramatic change in exhaust sound happend right before the light came on for the first time, and all the work to date has not changed the exhuast sound back to what it originally sounded like. This seems like this is a key clue in the problem and I'm wondering if I have a catlytic converter problem which is causing fouling or misfiring. I'm at the point of telling the mechanics to stop doing tuneups and parts replacements and look at the exhaust system - does that sound like the right direction to try?