I guess I will write down the experience of my attempt in fixing a Mazda 6. I got it second hand more than a year ago. The engine has been rough since I bought it. I had a dealer to look at it just after I bought it. They mentioned that I should replace catalytic converters. I didn’t do it as the job would be more expensive than what the car is worth now.
Trying to fix the Mazda finally became a side project once I find time. I first found some vacuum leaks from a vacuum line and intake manifold. I have those fixed almost a year ago but didn’t find time to continue to play with it. Power has been better now but idle seems to be getting even worse. Occasionally the engine just stalls when I press the break. The engine will not stall if I put in park or neutral though. I was thinking throttle body could be a culprit. But it doesn’t help after cleaning it. The car used to have some codes but all codes are gone now.
I am suspicious of vacuum leak. After all, this model seems to have lots of such problems. I tried to spray some throttle body around vacuum line to see if engine rpm changes but noticed nothing. Even worse, the spray seems to get something burning behind the engine. I put it off with an extinguisher and it doesn’t seem serious with only a little smoke. Hopefully not much further damage was introduced.
Someone suggested to let the engine run idle to have the computer to learn whatever after all the fix. I guess I will give a try when I am tired from work. If nothing works, I guess I will just have to take it to the dealer again for another inspection. Hopefully, this time they will find the source of the problem.