I have a 2003 Mazda6i 2.3 This car has run rough over the last year and I have not been able to fix it. Mechanics (Mazda dealership and general mechanice) have just reset the codes and fixed the usual suspects (non-existent vacum leaks), problem remained.

Here's what is does runs rough especially at idle, smells very rich when first start, it can (rarley but suddenly run a somewhat smoother once it is over 40 mph), low power. The codes have varried over the past year but usually to do with a misfire, lean bank, mass airflow sensor, cam sensor. Right now the codes are P0300 and P0340.

What I have done replace coil, plugs, wires, mass air flow sensor, CPM Sensor, checked EGR Valve, vaccum leaks all fixed. Considering O2 or Cat??

Any help would be appreciated.

 

Continue with a throttle body cleaning and cleaning the EGR orifices in the manifold. How can you be sure all vacuum leaks have been caught? My protege has a EGR boost sensor sitting on the firewall, passenger side. From there I followed a vacuum hose to the intake manifold and pulled it from the nipple. The nipple was completely plugged with deposits. After scraping it out with a wire and replacing the vacuum hose my CEL went out. Interestingly, I never had an EGR related code. I bet your 2.3 is similar.
It could also be fuel system related. Maybe the fuel pressure regulator is bad?