Need help with stutter and stalling issue

[lowridertk]

2012 Mazda5 AT. It has been running fine up till now. It started when it was about 1/4 tank of gas and filled up at a station in the city. shortly after it started bucking/ stuttering and when i come to a stop it would stall. so at this point i am thinking bad gas or fuel pump (original fuel pump) no codes. so i replaced the fuel pump (delphi brand) and replaced the gas and no change. at first when cold it would idle ok but once warmed up it would hesitate and stall even if i try to idle it at ~3000 rpm's.

Eventually I did get a P0300 so i replaced the plugs and swapped out coils and injectors with known good and cleared codes. again no change and no codes. on top of that it seems like it doesnt matter if its hot or cold. looking at live data rpm's, coolant temp sensor, STFT is about -5 but to me everything looks normal. i am at a lost with the lack of info.

Any help would be greatly appreciated.

 

[wlong01]

I'm not familiar with the M5's but did you check the fuel filter? Sounds like a clogged filter but could be other things and just a coincidence that it started after filling up.

 

[lowridertk]

Last I checked, it does not have an inline fuel filter.

 

[dlarkin_dc]

IME Stalling while idling is a dirty MAF sensor. Give the throttle body a clean and verify its working well when your in that vicinity.

 

[Lazy2.5]

I suspect fuel injector(s). you can verify based on fuel dilution level in oil , simple sniff test or even an analysis, or step outside of the car when the engine is warm and look for strong fuel smell

 

[lowridertk]

Ok found out what the issue was and could not been more simple. while i was swapping out the crank pos sensor i noticed the wire had a divot. upon further inspection it had rubbed on one of the pulleys or the belt exposing the wire. only 3 strands were still connected which allowed it to somewhat run and show rpm's on the live data. after repairing the wire it runs and drives so far without issues. all i can say is wow what a coincidence on when it failed and had me chasing a gas issue.

 

[MarioT]

Ok found out what the issue was and could not been more simple. while i was swapping out the crank pos sensor i noticed the wire had a divot. upon further inspection it had rubbed on one of the pulleys or the belt exposing the wire. only 3 strands were still connected which allowed it to somewhat run and show rpm's on the live data. after repairing the wire it runs and drives so far without issues. all i can say is wow what a coincidence on when it failed and had me chasing a gas issue.

Side question about the Crank sensor.
Does replacing it require a relearn procedure, or just plug n' play?

I got an intermittent no-start/wrong timing (like firing order stalls the starter instead of catching) and willing to bet on the sensor giving a false reading. For now, I'm just keeping the part ready for when the issue turns permanent.

 

[lowridertk]

Side question about the Crank sensor.
Does replacing it require a relearn procedure, or just plug n' play?

I got an intermittent no-start/wrong timing (like firing order stalls the starter instead of catching) and willing to bet on the sensor giving a false reading. For now, I'm just keeping the part ready for when the issue turns permanent.

should be just plug and play. if you have no-start/wrong timing it sounds like timing may have jumped. have you done any engine work?

 

[MarioT]

should be just plug and play. if you have no-start/wrong timing it sounds like timing may have jumped. have you done any engine work?

No engine work at all. I'm not worried at all about timing, but it would be a good way to describe it. Following attempt works perfectly and it mostly happens around a very specific range of winter temperature. No DTC set nor pending ones.
Knowing there's no complicated relearn, I'll swap it before next winter.