agent00F

I have an early 2009 S2 GT with 35k miles (weekend car), and recently the rpms would drop intermittently at idle. When it first starts, idles at 1200 or so fine, but once it warms up a bit idles goes to 1k and would repeatedly dip to 750 or 800 and back up. No real rhythm to it.

I've searched and some say to replace the coils/ignition, but also another post saying the "B" revision coils that 2009's come with don't need replacing early. Would appreciate any advice, thanks.

Loki

iTrader: (1)

800 is the normal idle speed. If it's dipping here and there, it could be due to A/C. All cars with A/C do this, when the compressor kicks in it puts extra drag on the engine. Turn off the A/C and see if it still does it?

As far as coils: there is no magic bullet with stock coils. B and C coils are better than A, but they all need replacement eventually. How old are they? 35k isn't terrible but you may want to plan preventative replacement anyway. Sometimes the first sign of coil failure is cat or engine failure.

agent00F

Ok, maybe I'm slightly off and it's going from 800 to 700, but it's definitely dipping/sputtering and you can see it on the needle, almost like a misfire. When it's happening it's going on almost continuously up/down once every sec or two, and not just occasionally like if A/C or other accessory turning on/off. Sometimes it doesn't happen for who knows why.

Forgot to mentioned a similar but more minor feeling sputter/misfire also happens at high throttle around 3k or maybe bit less but clears up >4k. Otherwise no issues. The coils are still the factory ones, so older in years but not miles. Car is otherwise is great condition, 3k oil changes and brought to redline at least every other drive. I don't mind buying coils but don't want to just throw parts at the prob. Somebody mentioned OBD monitoring and I have the blutooth adapter, but not sure what to look for specifically.

CaymanRotary

I find minor changes in RPM at idle pretty normal. Thats just how these engines are. Could just need new spark plugs. Only way to know is to take them out and inspect. Coils need to be bench tested. But if you arent getting any codes for the coils they are still working and won't cause damage to the engine.

Loki

iTrader: (1)

If you can read OBD, look for data on airflow,short and long term trims and rpm. When the car is warmed up and idling the fuel trims should be near zero. If they're not that might be a clue.

agent00F

Time for an update. I took the car for an oil change at a local mazda specialist place (they had 2 rx-8's in the back), and the owner said he wasn't sure the issue but doesn't look terminal. Then I swung by a rotary engine rebuilder to get some pre-mix and the builder there said to clean the MAF, so I did that and it seems to clean up some of the minor "misfiring" at low rpm full throttle, but didn't resolve rough idle. So I tried looking up data via ODB2 and this is what I saw:

Rpms: during idle bounced around in the 800's, will sometime dip to the 700's idling after spirited drive
Stft: bounces around -1% idling
Ltft: 6.2% idle, 4-10 while driving around
AFR: reaches plateau of 15.2 idle, 13.x light throttle, 14.x most of the time driving on throttle. Does have bit of a sawtooth pattern between small differential values (like jumps between 14.3 and 14.4)
Maf: just under 5 idle, of course more driving. After spirited drive idle will tend to [size=13px]fluctuate in waves between 4.5 and 4.7[/size]

I will also upload a vid sometime of the idle. It's not too bad on startup after settling, but kind of sputters periodically after a spirited drive.

I've also bought the platinum plugs on amazon and managed to borrow a jack, since it looks like it's coming up on the interval to change them anyway (car just under the 36k's).

Appreciate the advice here.

Loki

iTrader: (1)

You might have a minor vacuum leak. 4.7 is a bit low on airflow for 800ish rpm, and the ltft is a bit high. You have unmetered air getting in somewhere, but little enough that the computer can compensate with fuel trim. Loose or cracked hose, or a poorly sealed joint somewhere after the MAF.

Cleaning the MAF may have helped, or simply taking it out and reseating it may have closed the gap partially. Perhaps there's a poor seal there.

Regarding coils, I wouldn't take anything for granted. Some are better than others, but no Mazda coils are bulletproof. You can try to bench test them or replace preemptively just in case. That may not be your issue, but that's the thing with coils -- often the first noticeably symptom is loss of compression.

agent00F

So after many trips to home depot for socket extensions and universal joints, I got the plugs replaced and it fortunately resolved the issue. However the plugs looked pretty bad with different pattern of deposits on each one (incl one with what looks to be oil), so I started a thread on reading the runes plugs: https://www.rx8club.com/rx-8-discuss...2/#post4933514