Thank you for the old school picsHere are some old-school clues from Haynes Manuals (NA/1998 and scooter/2014).Is there a new set of photos for plugs off Skyactiv engines?
Strange 2.5T Engine misfire problem w/multiple choice answer game. Come play.
- Thread starter Tchman2016
- Start date
Link to video showing major vibration from misfires
https://www.dropbox.com/scl/fi/h0k8t4venrko64ml9wfjy/20260329_144233.mp4?rlkey=6erqnmbe4aeg8ys95iaqr6ot4&st=nxcses06&dl=0
https://www.dropbox.com/scl/fi/h0k8t4venrko64ml9wfjy/20260329_144233.mp4?rlkey=6erqnmbe4aeg8ys95iaqr6ot4&st=nxcses06&dl=0
N7turbo
2023 CX-5 2.5T ... 2024 MX-5 RF GT
That looks and sounds rough. If you bring the revs up, does it smooth out or no?
Yeah its terrible. That video is with the car in drive and foot on brake. If I take my foot off the brake the revs go up and it smooths out a good amount. If I have it in park and rev up the rpms it is smooth. It's really strange. Maybe I should just replace all four plugs and all four coils. I don't know if it's brake booster related anymore or some other issue but this is so frustrating. Still working on finding a coolant system pressure tester loaner from an auto parts store.
have you check your gas. have you change where you buy your gas. Or maybe you got a bad batch from Iran. LOL
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N7turbo
2023 CX-5 2.5T ... 2024 MX-5 RF GT
Well most issues mentioned in this thread should affect the engine at all RPMs. I don't recommend replacing the coil packs. They rarely go bad, and the odds of all four of them being bad are almost zero.
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- 2017 CX3 AWD
There has got to be some significance with the brakes because of the different response that you noted. Does your OBD tool show any brake-related sensor/stats or do you know all that is involved in braking? Mine shows two: brake fluid pressure and brake switch, but there could be more on your CX-9. What other aspects of the car need to react when the brake is depressed? Asking because I don't know. 
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- Mazda CX-9 Signature
N7turbo
2023 CX-5 2.5T ... 2024 MX-5 RF GT
Confirmed today that this is how mine behaves, minus the running like crap.
I checked coils before removing plugs and confirmed all tight. No codes in the ECM which is pretty ridiculous. Cylinder 3 ( second plug from the top in the pic I posted) had the most number of misfires but all cylinders had misfire counts. This just started out of nowhere. I did fill up a few gallons at Costco right before this happened but I think that is coincidence as you can see from the plugs this had to be slowly happening but not noticeable until now. I did fill up again with 93 octane from another station since and no difference. I dont know what's going on but it's really frustrating.
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Thank you! Ok so this goes deeper than a brake booster vacuum leak. I am determined to figure this out!
That's what I thought but apparently it's a normal thing confirmed by N7Turbo. I also tested my wifes 2021 CX-5 2.5T and it also lowers the idle when stepping on the brake while in drive and then idle raises when foot off brake but hers doesn't run like crap when I test it.There has got to be some significance with the brakes because of the different response that you noted. Does your OBD tool show any brake-related sensor/stats or do you know all that is involved in braking? Mine shows two: brake fluid pressure and brake switch, but there could be more on your CX-9. What other aspects of the car need to react when the brake is depressed? Asking because I don't know.
Scan tool data shows brake switch, pressure etc all normal.
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Update, performed cooling system pressure test cold and let sit for 1 hour. Pumped to 16 PSI and pressure dropped .75 of 1 psi during that time so was down to 15.25. I dont know if this is significant or not but its something. Also I don't know if it's my imagination or not but I can swear I smell the slight odor of coolant every time I drive the car and check under the hood when it's warm.
After the pressure test, I swapped the number 3 cylinder coil with my wifes car since thats the one with the carbon fouled looking plug. I started her car and used the scan tool to check for misfires. None. Also ran fine. So I swapped it back.
Then, I replaced all 4 plugs with new identical NGK laser iridium from advance auto and checked the gap before installing. They were right on .030 from the factory. I also swapped the #3 coil with #1 coil at the same time. Put everything back and fired it up.
Result was very smooth on startup and idle in park. Put into drive with parking brake on, idle dropped a bit but still seemed smooth. Applied brake pedal in drive and idle dropped more and engine began to sound and shake worse but not as bad as before the plug change.
Checked live data for misfires and there were a handful on 2 of the 4 cylinders.
At this point what would you do next?
It's driving me nuts!
After the pressure test, I swapped the number 3 cylinder coil with my wifes car since thats the one with the carbon fouled looking plug. I started her car and used the scan tool to check for misfires. None. Also ran fine. So I swapped it back.
Then, I replaced all 4 plugs with new identical NGK laser iridium from advance auto and checked the gap before installing. They were right on .030 from the factory. I also swapped the #3 coil with #1 coil at the same time. Put everything back and fired it up.
Result was very smooth on startup and idle in park. Put into drive with parking brake on, idle dropped a bit but still seemed smooth. Applied brake pedal in drive and idle dropped more and engine began to sound and shake worse but not as bad as before the plug change.
Checked live data for misfires and there were a handful on 2 of the 4 cylinders.
At this point what would you do next?
It's driving me nuts!
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- 2017 CX3 AWD
Hang in there you've made great progress in two days!
