Suspected misfire - idle log
 

Does this look like an intermittent misfire or weak fire to you? This is a log from MazdEdit. This is 70 second log at idle, fully warmed up. Stock car, stock tune (has never been tuned, always stock). I replaced the coils about 7k miles ago along with the plugs, using OEM. MAF is clean.

At idle, the exhaust will intermittently sputter (like a very quiet pop/burble), which is usually accompanied by a short RPM dip. These RPM dips are slight, causing RPM's to fall from ~825 to the high 700's, and only last a second.

I noticed that these RPM dips are accompanied by a short spike in AFR, which I'm thinking could be due to uncombusted fuel being picked up by the o2 sensor. My stable idle AFR's are ~14.8, and these spikes run up anywhere between 15.2 and 15.6. They register 1 or 2 tenths of a second after the RPM dips.

The RPM dips are also followed by brief spikes in my short term fuel trims. My long term fuel trim is +0.78 at idle, and short term is usually stable at about +2. The spikes run up to about +4.5.

So i'm thinking this is a bad coil. Does anyone have a different assessment? Anything else I should do to test.


https://cimg8.ibsrv.net/gimg/www.rx8...66bcd08d29.jpg

https://cimg6.ibsrv.net/gimg/www.rx8...3e3f846f39.jpg

My car has the exactly the same symptoms, Im interested to see what you find out.
I also have the error code for rear rotor misfire too.

Well your case is very clear cut. You have a misfire. Check to make sure your wires/connections on the coils and plugs aren't loose. If they are OK, you have bad coils. Need to replace them. A misfire can do damage to your motor so I'd do it right away.

Thanks for that, I have a set of LS2 coils, new leads and plugs, just waiting on the harness to arrive then Ill change them.

Ok well I exchanged all four coils and plugs under warranty (advanced auto parts, intermotor brand, no questions asked).

I'll install the new coils/plugs tomorrow and report back. Think I should probably order some new wires too and replace those at the same time.

Woah - slow down with the feedback everyone :smoker:

An update: replaced all four coils, plugs, and leads. Two coils looked like they were on the road to going bad (both were leading coils) but my plugs looked OK. Changed my air filter at the same time (mine was 75k miles old I believe, swapped in another OEM) and re-cleaned my MAF for good measure. While cleaning my MAF, I did notice that the o-ring was slightly deformed (slightly flattened rubber on a small portion), so I replaced it. I also sprayed carb cleaner around the vacuum system to check for any leaks, didn't find any.

Unfortunately, the issue still persists. The car did regain quite a bit of power according to MazdaEdit's built in dyno - had been reading ~163 crank and now ~195-200 crank. Car pulls a fair bit harder so I think the improvement is real. Exhaust temperatures are also quite a bit lower, peaking at ~1660* on WOT pulls when they used to climb past 1730*

But...i still get the same sputtering and popping at idle. Subjectively, it almost seems a bit worse (i.e. more frequent). Long term fuel trims at idle are also high (+7.8) with short term bouncing around between +1.5 and +2.5). Standing behind the tailpipes, I can feel a small burst of air/pressure coming from the exhaust with each pop.

Otherwise the car runs well. Power feels quite good and there are no misses all the way up to redline. AFR's are flat at 12.5 after 5k RPMs (again, car is stock and never tuned), which is a considerable improvement from before. AFR's used to jump up and down between 11.5 and 13.5 over the same RPM range.

But clearly there is something wrong considering the idle/popping.

I took a video and uploaded it to youtube. Pops occur at :03, :07 (small), :10, :11 (small), :13, :17. Engine is fully warm (~180) in this video.

Let me know if there is anything else I can record to help diagnose.

Suspected misfire - idle log
 

Is your engine warm or cold when this happens?
What method are you using to verify that your coils are good/bad?

Engine is fully warmed up. Only method I used to verify the coils was replacing all four with brand new ones (as well as NGK leads and new plugs). I suppose the new ones could also be bad but I doubt it. Maybe the harness? Don't know how i could test this though.

I have a multimeter. And a timing light somewhere packed away. Cheap timing light though, don't know if it would still work. Any other ideas on testing the ignition system?

Suspected misfire - idle log
 

https://www.rx8club.com/series-i-do-...-wires-222641/

Is it possible you have an exhaust leak?

I don't think i do but anything is possible. Would an exhaust leak cause popping/sputtering like that? As I mentioned above, the pops are accompanied by a small plume of pressure shooting out the tailpipe. You can feel it sticking your hand out behind the tailpipes.

Out of curiosity, why does a leak in the exhuast system cause these symptoms?

My S2 always had an uneven idle, with some popping in between from the day I purchased it. I think it's just the nature of a rotary engine?

However, I'm seeing ~10.75 afr 7k onwards although we could be looking at different PIDs or the S2s could have been made to run that rich to extend the life of the cat? I'm also seeing high (~6% LTFT since replacing the coils [Napa Echlin] / plugs about 2k miles ago) for some reason

Popping and sputtering is normal, but your ltft seems a bit high. Higher usually an air leak. It's possible that you're experiencing some weird readings at your rear O2 sensor, which could be related to a small exhaust leak (ie the gasket). It's not a defintive answer, but I'm curious about it also.

TL;DR

Do you have a stock intake?

Per my above post, car (including intake) is completely stock.

Interesting you mention rear o2 sensor. I have gotten an intermittent p0037 code, heater sensor voltage low on rear bank o2 sensor. It goes away, and I always understood the rear o2 sensor wasn't important. I suppose I was wrong about that. May just replace it with a denso.

Have you checked the screens in the intake to ensure they are intact? There should be two IIRC.

The rear O2 sensor isn't important and the popping/pressure at idle is completely normal.... I find it happens more in colder weather.

Edit: I see the exact same thing on my AFR gauge as you too.

Yes, inspected the airbox and both screens are intact and clean.

And that's what I thought about the rear o2 sensor. There has been a fair bit of misinformation contributed in this thread - wonder if a mod could redact it to prevent confusion from future readers encountering similar symptoms.

So...I'm satisfied that the popping is normal based on feedback from you guys. Now the only issue I need to resolve is the low MAF flow readings at idle. They get to ~220g/sec at WOT high RPM, but wonder what's going on at idle.

Your MAF readings look fine at idle. 5-6 G/s is normal. the dips in the idle are common, but a properly running stock 8 doesn't do that. Is it a real problem? Not for most but I am OCD so stuff like this must get resolved, :lol:

Im the same way 9k. This is driving me nuts despite the fact that the car drives well otherwise. So i want to figure this out.

My maf actually reads around 4.7 at idle so its low.

Today has been warmer, around 55* as compared to 30 yesterday and the popping is less frequent but still there. WTF?!?!

Dips to 4.7 or averages? It will dip if the idle speed dips. Have you done a vacuum reading on your engine at idle warmed up?

Also that graph style is tough to read but what is the average of your STFT at idle? It should average within +/- 3% of 0.

Suspected misfire - idle log
 

There is no misinformation. Look up that definition. It was a question that I was curious about, which has yet to be ruled out.
But if you want to rule it out completely, that's your choice.

I was referring to the point about rear o2 sensor readings contributing to any of the symptoms. They would not.

Average is 4.9 g/sec or so.

Short term fuel trims are approx +2.5 at idle but long term fuel trims are +7.81 at idle...so quite high.

I tested my ignition system tonight and it is very strong. Set the tester to a full inch even and got a strong spark at all RPM ranges (in the thread linked by RX8soldier earlier, the test employed a gap of only 3/8 of an inch, so a full inch should definitely indicate strong coils).

https://cimg0.ibsrv.net/gimg/www.rx8...874c9ca258.jpg



I haven't done a vacuum reading - I'll search on how to do that.

No idea what's wrong with my car! The popping is driving me nuts. And its not just popping, it is accompanied by a dip in RPM's. If I hold the accelerator slightly and rev to ~2k rpms, the car will feel as if its missing every time there is a pop. Little dips in RPM's. Really feels like a misfire but clearly isnt.

Well both your LTFT and STFT should be within +/- 3% so you definitely have an issue. Could be a vacuum leak, O2 sensor degradation, fuel pump/pressure issue (actually test it), etc.