I recently bought a 2012 Mazda 3 i touring (refreshed). It has the standard 4 cyl 2.0 DOHC engine. It has an automatic transmission. The car currently has 350 miles on it. When I first start the car after letting it sit for a while it will idle at 1,000 rpms. However, After driving it for about 2 miles it will idle around 750 rpms (with slight fluctuation at times + or - 10 rpms). This happens both at a stopped position in drive, as well as in park. Along with the reduced idle it develops a slight dull knocking sound that seems to come from the engine compartment. If I rev the engine the sound goes away until it comes back down to around 750 rpms. I popped the hood while this was going on but couldnt pinpoint the sound, but the engine does appear to rattle quite a bit (compared to my last car anyway. A 2002 Buick.). Is all this normal during the break in process? Is this muffler, transmission, or engine related? Am I just freaking out or should I take it to get checked?

 

The increased idle at startup is normal. This is common in most fuel injected engines. However, I can't answer to the "dull knocking sound" you hear. You still have a lot of warranty left so if it does get worse, have it checked. In any case mention it during your first service visit.

In the meantime you may take a test drive in another Mazda and see if the same conditions are present.

virg

 

I was thinking that the idle was normal. Let me clarify about the knocking sound. It does not happen as soon as the car reduces to 750 rpms. It seems like it happens about 4 miles in to a trip, usually after I have sped up to 40 0r 50 mph and them come to a stop. Once stopped the sound slowly creeps in. I was thinking maybe it was something loose, but the loose piece would start rattling from the time I started the car. On a side note. When I pop the hood, the air filter boxes and the hoses around the box seem to vibrate alot. Is that normal as well?

 

I just picked up a 3i Sport this week and I've noticed the same sound from the engine bay with the same shaking from the air box and hoses leading up to the throttle body. Honestly, I think it's just the way the engine is. my brother has a '05 Mazda 3s and it shakes a lot at idle as well.

If I gets worse I'm going to take it in, but it only has 100 miles on it so hopefully once in breaks in, it'll stop.