To begin, my Protege has had a cat code on (P0400) for about a year, and just started driving poorly over the past month. Started by misfiring and throwing a P0304 code (had the ignition coil replaced...which was previously replaced about 4 months prior). Still ran poorly so we put dry gas in the tank in hope that it was just bad gas. After this it did run decent for about 2 weeks, then the problem started again. Check engine flashed repeatedly showing again a P0304 code. Turned the lights off and brought it back to a shop. They didn't replace the coil again, but just looked things over and said they couldn't figure out what was really happening yet and to bring it back when it gets worse. Basically the car runs decent until it is warmed up. At which point I hear a very faint popping sound through the air vents, and then when I reach a stoplight the RPMs bog way down, and occasionally the car stalls. When I start driving from the light the car feels like it misfires multiple times (not flagging any code yet though), but usually evens out until the next light. I don't have a ton of money or know much of anything about cars, and don't want to replace the cat unless I know for sure that it is the problem. Anyone have any ideas what could be causing this, or if this sounds like a cat issue? Thanks for the help!
Rough idle, misfiring, and stalling at stop lights. on 2003 Mazda Protege
by pipinl in Troy, NY on December 09, 2013
ANSWER by ziptie12 on December 09, 2013
Restricted exhaust due to failing (possibly melted down) converter?Temp. create an exhaust 'leak' in FRONT of the converter by loosening at a flange connection , and test drive . OR hook a vacuum gauge to a port on the intake (engine side of throttle plate)and watch the change in manifold vacuum from cold start til warm and idle to 2500 rpms.If vacuum starts dropping as it warms up maybe suspect exhaust restriction. Should be steady reading , if vacuum is jumpy suspect possible valve problem as cause of misfire.