Hey all,
Just wanted to get some input on an issue I'm having, although I believe I know where to look next.

I took the 8 for a test drive yesterday after doing an oil change and replacing the primary O2 and it started and idled fine cold. Went to take it for a drive and it stalled when returning to idle as I was coming to a stop once it was warmed up.
Started it back up and when I was accelerating it started stumbling and hesitating around 4000-4500rpm in every gear. Once it got to around 5000rpm and higher it ran and pulled just fine. Overall seems to idle fine, mostly steady aside from small dips/raises but that could be due to all trims being reset.

So the main issues are:
1. Stalling when returning to idle coming to a stop.
2. Pretty significant stumbling/hesitating at 4000-4500rpm under throttle(any amount of input)
3. Slight intermittent idle hunting, but could be due to trims being reset.

Recent items that were addressed and mods:
1. New genuine OEM MAF
2. New genuine upgraded starter
3. New battery
4. New NTK primary O2 (this was replaced due to a prior P0172 code)
5. SSV and ESS cleaned
6. New AIR/SSV/VDI solenoids
7. New plugs, BHR coils/wires
8. AEM intake installed prior to pimary O2 replacement
9. Battery connections checked and solid
10. Not running a catalytic converter

I believe that's all the relevant stuff that was replaced/addressed within the last 500 or so miles. I don't currently have any codes but it may not have had enough time and drive cycles to post any. In my mind, the next thing I need to look at as a culprit would be the fuel pump. Maybe the alternator, but that's less likely and I'll rule that out tonight when I go home and put the multimeter to it.

Does anyone have any other input as to what else it may possibly be or what to check other than fuel pump? It is a 2004 MT with a fresh motor rebuilt by Mazmart.

TIA!

 

Just a quick update, I took a really brief look yesterday to see if I had any vac leaks. My MAF flow rate is 4.9g/sec once warmed up at idle, so maybe a potential small leak. My STFT was -1.56% and LTFT was -7.03% once warmed up at idle.

AFR's at idle were 15.0, but would lean out to 16-17 when I gave it a little throttle, which was weird since the pending P0172 and negative fuel trims would indicate it's cutting fuel due to running rich...so next order of business is to put a fuel gauge on the line and see what the fuel pressure is, check for vac leaks, and check to see if coils/wires are healthy.

It hunts and hangs when returning to idle, I'll upload a video and the quick snapshot I took of the data I got yesterday.

Any other ideas greatly appreciated.

 

3/16/2022 Update: So interestingly enough, all the issues were gone today... lol

All I did was clear all fuel trims/learned parameters using my OBD scan tool, which I thought I already did since the battery was disconnected and I reset the NVRAM, but nonetheless I drove it around today for around an hour and all the issues were gone.

Idle was great, AFR's were at 14.7 at idle and never went lean, they go to around 12.3 at WOT, STFT and LTFT were all steady around 0 for the most part with only slight changes to add/subtract fuel mainly on the LTFT.
No more stumble at ~4500 RPM, no more stalling returning to idle. All in all, the car ran amazingly well and strong.

I guess it doesn't rule out intermittent fuel pump failure, so I'll keep an eye on that and do some further testing, but as of right now it seems as if the magic rotary fairies snuck into my car and fixed everything lmao.

3/17/2022 Update:

Well, it's definitely intermittent. It was fine yesterday, and fine again today on my way to work. Ran amazing. Then tonight it started doing it again halfway home from work.

Breaking up and stumbling starting ~4000-4500rpm, stalling when returning to idle as I came to a stop. I was monitoring AFR's this time around and as soon as it started happening the AFR's went to max rich.

Off throttle it was 20.4, which is fine, but as soon as I gave any throttle input it instantly went pig rich and stayed pegged at 11 regardless of whether I gave it 1% or 100% throttle. STFT and LTFT also stayed at 0 the entire time, which was interesting, because it wasn't trying to cut back on fuel whatsoever.

So now I'm leaning towards maybe I got a bad primary O2? I know O2 sensors sometimes can get "lazy" and work only when they want to. It's a brand new NTK sensor, so I may get an actual OEM Denso unit and see if that makes any difference.

If anyone has any thoughts they'd be appreciated.

 

Following this thread as I'm having similar issues after my battery ran flat. For me it is mostly only stumbling at 4250-4750rpm and only under very light throttle. WOT you wouldn't know that there was an issue.
idling not as bad as the video you posted but still pretty horrid dipping into 600s rpm before jumping back up. Going to try resetting the nvram today as hopefully that is just it.
​​​Good luck

 

My idle issue is gone, I think it was just a matter of relearning fuel trims and whatnot.

The stumble and the AFR's only reading full rich on throttle and full lean off throttle is still there though. Ordered a new OEM primary O2 and will install and see if it goes away.

 

Sometime ago I read that RX8 are very sensitive to non-OEM O2 sensors. Maybe if you have access to a buddy that can lend you a known good O2 before breaking open your piggy bank!

 

I did the same thing a while back. Bought the ntk o2 sensor and it was lazy. Changed to the OEM Denso $$$ and fixed the issue.

 

I'm having an issue with the car not starting once it's driven. It still is a little hard to start first thing in the morning as well but when I stop I have to turn it over constantly and eventually it will restart do you think the O2 sensor that you mentioning above could do this

 

No the O2 sensor isn't turned on while the car is starting. Hard start after driving is usually poor compression, you'll want a compression test from a place with a rotary tester. Mazda dealer is one, rotary specialist is another.

Could also be poor ignition.

 

Thanks. I replaced the coils and wires. I need to check the plugs. I've noticed that if I hold a steady RMP/speed the car has a shudder. Sometimes at a light it will just shut off. It feels like fuel more than anything. I checked the fuel pressure (blue line) and there is pressure. I'm just not sure as to how much being my gauge stops at 10 PSI. Its got a lot of pressure though because like a moron I turned the key on and gas shot the rubber gauge fitting off and shot gas straight out of the line. I didn't know that it was 54-62 psi required. I'm going to have the fuel pump pressure checked at a shop.

Being the O2 sensor isn't on during cold/hot starts I'm thinking maybe injectors. Are there any sensors that would create a hard start? I've been told Camshaft position sensor? BTW idle is all over the map....it surges.