Strange 2.5T Engine misfire problem w/multiple choice answer game. Come play.

[Tchman2016]

Here 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?

Thank you for the old school pics

 

[Tchman2016]

Link to video showing major vibration from misfires

https://www.dropbox.com/scl/fi/h0k8t4venrko64ml9wfjy/20260329_144233.mp4?rlkey=6erqnmbe4aeg8ys95iaqr6ot4&st=nxcses06&dl=0

 

[N7turbo]

Link to video showing major vibration from misfires

https://www.dropbox.com/scl/fi/h0k8t4venrko64ml9wfjy/20260329_144233.mp4?rlkey=6erqnmbe4aeg8ys95iaqr6ot4&st=nxcses06&dl=0

That looks and sounds rough. If you bring the revs up, does it smooth out or no?

 

[Tchman2016]

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.

 

[tony357]

Good evening all,

Here's an interesting one I thought I'd post to get a consensus on an engine misfire on all cylinders issue on my 2020 CX-9 GT 2.5T. 36k miles.

Yesterday, I noticed the engine running rough, lower idle than usual with the car in drive and my foot on the brake. Engine sounded wierd and was stumbling and felt like it was close to stalling. If I remove my foot from the brake, the idle went back up by at least 100 rpm and the engine smoothed out. Press again and idle lowers and engine stumbles.

This does not happen if the car is in park idling, even with foot on brake. It only happens when in drive or reverse with foot on brake.

Broke out my TopDon BT scan tool to look at live data stream and confirmed that there were many misfires on all cylinders occurring with numbers slowly rising, however NO check engine light was on or had been triggered thus far. This was confirmed over multiple trips. Incidentally, if I leave the car to cool off and come back to start it a couple hours later, the engine has trouble starting for a few seconds it cranks and stumbles and finally gets going. Almost as if there was bad plugs or coils, but that is not possible because the misfires are across all cylinders.

I looked under the engine cover and there is no damage to the wires to the coils or injectors from any animals.

Further, the plugs were replaced with genuine NGK at 30k miles as preventative maintenance even though 40k miles are the normal time to replace.

So, before I take it to the dealer(I have a mazda oem extended contract) I thought I'd post and play a game of what could be wrong and who votes the most on the multiple choice answers to the problem below.

Possible choices are:
A) Bad brake booster with torn diaphragm/vacuum leak letting unmetered air into engine causing misfires and idle fluctuations when brake pedal applied under engine load

B) Dirty or stuck EGR valve

C) Cracked Cylinder head causing coolant intrusion and misfiring in all cylinders,also causing hard starting since coolant can pool in the cylinders and is not combustible like gas.

D) One or more cylinders have low compression due to bad rings or for unknown reasons.

E) Combination of any of the above

F) None of the above.

Lets go!

have you check your gas. have you change where you buy your gas. Or maybe you got a bad batch from Iran. LOL

 

[N7turbo]

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.

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.