Hi I have a 2008 mazda 6. On cold idle the rpm is fine 800rpm. When the car warms up it goes down to like 700rpm+ which is still fine. The problem I have is that if I turn on the headlights the engine sounds like it wants to shut off and the rpm goes up and down 600+ to sometimes 580rpm. If I press the power window switch it does the same rough idle. If I press the AC it goes back up to 700+rpm. I check the battery it is 12.5v I put in another battery and it's doing the same thing. The alternator shows 13.5v when it's not under load when I turn on the heat lights I've seen it drop to as low as 12.9 but then fluctuate upon down in the 13.0 to 13.5 range. I checked the alternator I connect the black lead to negative and positive lead to alternator I get 0.05 I put the positive lead on all ground and I get 0.05. I check headlights 15a fuse I get 0.02 resistance. I connect to a scan tool it shows engine load value going up higher when headlights are on. I also see vacuum value go lower. What could be my problem?

 

I had the same problem on my '09. When turning the heater on high the headlights would dim and idle would flutter when stopped at a light and almost stall. Replaced cabin fan, then alternator and battery as they were old anyway. Headlights stopped dimming. Idle would still flutter and it actually stalled. Then cleaned throttle body and MAF sensor, changed air filter and idle flutter is now completely gone.