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Misfires on cold starts with no CEL

Parcham79

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Glen Cove
#1
Hey guys, I have a 2017 ST with 12,000 miles on it. I'm currently towards the end of my tuning process with Mountune and have begun to develop misfires only on cold starts at idle. I receive no check engine light but you can audibly hear the strong "popping" from the exhaust that was never there before. Car runs great and pulls through the gears very well and idling while warm is also no issue. AFR during cold idle hangs around 15.5 and stays there until about 30 seconds of road driving before returning back to 14.55-14.7 . Tomorrow I will pull all plugs and inspect their gap and condition and I will then do a compression test (wish me luck). If the issue is not resolved is there anywhere you guys can point me to for these symptoms, couldn't find anything else on the forum even after quite a bit of searching that fit these specific symptoms...

Modifications
-Whoosh Catless DP
-Mountune Panel Filter
-Turbosmart ATM BOV
-Radium Engineering PCV Catch Can
-Cobb AP with Mountune 93 oct tune
 


TyphoonFiST

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Rich-fizzield
#2
Hey guys, I have a 2017 ST with 12,000 miles on it. I'm currently towards the end of my tuning process with Mountune and have begun to develop misfires only on cold starts at idle. I receive no check engine light but you can audibly hear the strong "popping" from the exhaust that was never there before. Car runs great and pulls through the gears very well and idling while warm is also no issue. AFR during cold idle hangs around 15.5 and stays there until about 30 seconds of road driving before returning back to 14.55-14.7 . Tomorrow I will pull all plugs and inspect their gap and condition and I will then do a compression test (wish me luck). If the issue is not resolved is there anywhere you guys can point me to for these symptoms, couldn't find anything else on the forum even after quite a bit of searching that fit these specific symptoms...

Modifications
-Whoosh Catless DP
-Mountune Panel Filter
-Turbosmart ATM BOV
-Radium Engineering PCV Catch Can
-Cobb AP with Mountune 93 oct tune
Video would be great for noise....If there is any type of misfire the vehicle will register it in the computers memory and produce a code. The vehicles ignition would breakup and the car would run awkward and shake sometimes violently. I think what you are experiencing is due to the cat-less DP and some backfiring...which some stock Fists and owners claim with no mods to the vehicle have experienced. But have Mountune take a look at the data as they should be able to tell you why its going on.[party]
 


Quisp

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Davenport
#3
Check your coolant level and check the exhaust smell on startup for that sweet nectar of antifreeze. Hopefully not.
 


Sekred

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Mid North Coast
#4
Hey guys, I have a 2017 ST with 12,000 miles on it. I'm currently towards the end of my tuning process with Mountune and have begun to develop misfires only on cold starts at idle. I receive no check engine light but you can audibly hear the strong "popping" from the exhaust that was never there before. Car runs great and pulls through the gears very well and idling while warm is also no issue. AFR during cold idle hangs around 15.5 and stays there until about 30 seconds of road driving before returning back to 14.55-14.7 . Tomorrow I will pull all plugs and inspect their gap and condition and I will then do a compression test (wish me luck). If the issue is not resolved is there anywhere you guys can point me to for these symptoms, couldn't find anything else on the forum even after quite a bit of searching that fit these specific symptoms...

Modifications
-Whoosh Catless DP
-Mountune Panel Filter
-Turbosmart ATM BOV
-Radium Engineering PCV Catch Can
-Cobb AP with Mountune 93 oct tune
Are you sure the AFR is around 15.5 at cold start?. That sounds a little lean to me. I would expect to see around 12.0 during a cold start or richer depending on the ambient temp. Popping from the exhaust can indicate lean AFR misfire.
 


A7xogg

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Hampton roads
#5
Mine also idles at that afr on coldstart(completly stock exhaust) so im thinking carbonbuild up
 


OP
P

Parcham79

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Glen Cove
Thread Starter #6
Hey guys so I pulled the plugs and they all looked great and were all gapped accordingly still at 0.026” (as I expected they only have 1,000 miles on them). I was unable to get hold of a compression tester yet though. I took a video but won’t be able to upload it until later tonight. Is there a chance it might be my muffler going? Just a thought...
 


OP
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Parcham79

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Glen Cove
Thread Starter #7
No sweetness thankfully ha, but I’m not ruling anything out till I get that compression test done
 


alexrex20

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Spring
#8
Sounds like an exhaust backfire, not a misfire. Misfires would be picked up by the computer and set a CEL.

Ditch Mountune.

Sent from my SM-G950U using Tapatalk
 


OP
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Parcham79

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Glen Cove
Thread Starter #9
Hey guys sorry for the late response but I ended up doing a compression test and the numbers were great.

Cylinder 1 - 164 Psi
Cylinder 2 - 164 Psi
Cylinder 3 - 165 Psi
Cylinder 4 - 162 Psi

Still no check engine light and car is still running great while driving. I will occasionally have my idle dip down to about 650 RPM which is noticeable but the car always quickly catches itself. This only happens about once per 30 minute drive so it's not a crazy frequent event. Running out of ideas as to what this may be. Maybe a leaking injector? Just had the thought since the problem doesn't occur with throttle but only at idle.
 




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