qwerty210

Hi there,
Im posting about a rough idle on my 04 Mazda rx8 with 90000 miles on its first engine. In my 2 years owning the car I have done my research and learned so much about the engine and the car. The car has a aem cold air intake and a custom catback exhaust as far as engine mods go. I have had a on going issue with a rough idle for the last number of months. The fuel economy on it is worse than usual right now at around 14 to 15 mpg not sure if this is related to my rough idle or not. After searching the forums I figured there was a good chance that the idle was related to my cold air intake having the screens sucked out of place and such. I have played around with the screens on the intake on and off for the last couple months trying to get them into place and have had no luck changing the idle at all. At this point im sorta doubting the intake has something to do with it but im open to all suggestions. First thing I replaced is plugs coils and wires. This did nothing for the rough idle. After that afraid of engine failure I got the engine compression tested by my local mazda dealer and the compression checked out fine. I told them about the rough idle and they pointed to the cold air intake right away. Currently im thinking it could have something to do with a o2 sensor but there is no cel or possibly motor mounts however I don't see how motor mounts could make the idle lower. As far as the rough idle goes it normally happens at stop signs once the car has been on the road for over 10 minutes or so. The car idles kinda low most of the time at around 850/900 rpm but dips down lower than that sometimes almost to the point of stalling (although it has never stalled) causing the whole car to vibrate which is quite annoying especially when I have friends in the car then pops back up to the 850rpms. Im willing to get a new o2 sensor to test if that is the issue however I just wanted to get some opinions before spending antoher $100+ on my rough idle.

RIWWP

iTrader: (2)

Correct, the AEM intake by itself won't cause the rough idle. I bet if you asked the dealer if the Mazdaspeed CAI made the car idle rough, they would say "nope!", even though they are exactly the same (Mazda just rebranded the AEM, literally identical, same production line and all)

Correct that motor mounts won't cause an idle fluctuation. It will feel rough, but if motor mounts were the only issue, the needle would remain steady despite the vibration.

Good that you already replaced coils, plugs, and wires.

What were your compression scores? "Fine" isn't a set of 7 or 8 numeric numbers, which would have been your right to have from the dealer's test.

Some more easy / cheap / free things to rule out:
- Clean the MAF with MAF cleaner
- Clean the ESS, and reset the learned profile
- Look for any obvious vacuum leaks
- Get an OBD2 bluetooth adapter ($25+, depending on what you get), pair it with your smartphone, download an OBD2 app, and look to see what your AFRs, fuel trims, and MAF is doing when the idle dips. If the STFT is +5% or more with the MAF below ~5g/s, you likely have a vacuum leak somewhere you haven't found yet. If the AFRs are all over the place, also likely a vacuum leak. If the AFRs are constantly lean, the front O2 sensor may be failing. If the AFRs are constantly rich, you may have a MAF fault.

04Green

iTrader: (1)

++ ^^^ that is most of what is in the $100 thread in my signature..

I suggest reading the rest after MAF adn ESS clean. Make sure car is cool. do 20 brake stomp, then start Let it idle until is re-learns everything (10 minutes). If you disrupt this process, life sucks. You may have to keep foot slightly on gas for the last part, or turn on AC to bump up idel.

Good luck.

qwerty210

Ok so compression test numbers I used the forum on here to check when I had the test done and they check out fine. Today I cleaned the maf and ess. The ess was very dirty. I did the 20 brake stomp and let it warm up then took it for a drive. The idle still sits low under 1000rpms at around 850-900rpms. I didnt get the chance to take it for a long drive though. I ordered a bluetooth obd2 and am going to check out that when I get it.

RIWWP

iTrader: (2)

I still don't see numbers here? Quite a few membes interpret wrong and then label their engine as "fine" when it clearly isn't. Or listen to techs who tell them it's "fine" just to avoid a warranty claim. Just trying to help make sure that they are actually good.



The idle is supposed to be at 850rpm, so there is only a problem if it isn't steady there. Periodically you will have a bit higher of an idle, like when the coolant temp is still cold, the AC is on, etc... But it should sit at 850rpm normally. If it is jumping around or dipping under that, there is a problem to address.

So if your complaint is that it is at 850rpm, there is no problem.