Misfires under boost

[mhierro235]

Good evening 10th gen owners. I am looking for some help with a misfire issue ive been having.

some details for context. 2017 civic si with around 90k miles, all stock except for a catless downpipe, and flashpro. The car was running great for a while on the +9 psi tune. when the misfire/breaking up started happening, it was taken back to the stock tune and the issue persisted.

since then i have replaced the spark plugs and the fuel injectors, however still misfiring. when the injectors were replaced it actually ran better and it actually would stay in boost. Until the next morning when the issue came back. I have taken a few datalogs hoping i could see something unusual. The only thing i can see is the A/F ratio fluctuate to 29 and back to 14 ish when the misfire is happening. this leads me to believe the car is cutting fuel when in boost or large load (i could be wrong). I also noticed the s.trim adding fuel when getting into boost. im pretty comfortable with working on the car, and i have gotten familiar with making changes to a tune.

I have a few datalog that ill attach to this post. if anyone could point me in the right direction , it would really help. i have read all kinds of threads talking about
head gaskets, coils, battery, spark plug gaps ect. instead of changing more parts im hoping the the logs could narrow it down. any help is appreciated.

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[speedyserg]

Good evening 10th gen owners. I am looking for some help with a misfire issue ive been having.

some details for context. 2017 civic si with around 90k miles, all stock except for a catless downpipe, and flashpro. The car was running great for a while on the +9 psi tune. when the misfire/breaking up started happening, it was taken back to the stock tune and the issue persisted.

since then i have replaced the spark plugs and the fuel injectors, however still misfiring. when the injectors were replaced it actually ran better and it actually would stay in boost. Until the next morning when the issue came back. I have taken a few datalogs hoping i could see something unusual. The only thing i can see is the A/F ratio fluctuate to 29 and back to 14 ish when the misfire is happening. this leads me to believe the car is cutting fuel when in boost or large load (i could be wrong). I also noticed the s.trim adding fuel when getting into boost. im pretty comfortable with working on the car, and i have gotten familiar with making changes to a tune.

I have a few datalog that ill attach to this post. if anyone could point me in the right direction , it would really help. i have read all kinds of threads talking about
head gaskets, coils, battery, spark plug gaps ect. instead of changing more parts im hoping the the logs could narrow it down. any help is appreciated.

High rpm misfires 5k+ could be a few things - your leaking coolant causing knock up top (headgasket)
Your hitting a fuel cut (clogged fuel filter)

Check your coolant levels and see if your eating it slowly. If you haven't changed your oem fuel filter it wouldn't hurt to change it while upgrading the pump at the same time 2 birds, 1 stone.
Gap the plugs a little tighter too.

 

[gtman]

OP, what did you gap the plugs to?

Are you getting P0300 code(s)?

 

[mhierro235]

Speedyserg -
I’ll recheck the coolant levels, I still get the misfires around 3k when demanding higher load. Would coolant still be able to be pushed into the chamber at this lower rmp??
I dont think the fuel filter has ever been changed, at least not while I owned the car, I will look into replacing. I’m guessing pump and filter are in the same location hence why you mentioned upgrading??

gtman-
When I replaced them I didn’t touch the gap, I put them side by side with the old ones and the gap was identical so I didn’t touch it to play it safe. I’ll gap them a little tighter and while I’m in there I’ll look for some coolant in the combustion chamber.

thank you for the help!! I’ll update in a few

 

[gtman]

gtman-
When I replaced them I didn’t touch the gap, I put them side by side with the old ones and the gap was identical so I didn’t touch it to play it safe.

That's a terrible way to check gap size. Remember old plugs tend to get a wider gap over time. For a +9 Hondata Si tune, I'd gap them .026"-.028".

One other thing, because I get swamped with requests, I generally only take the time to read datalogs from people who are part of my tuning thread (link in my signature) but I made an exception here.

First of all those are not good logs at all. You basically weren't even driving like grandma. I think a bicycle would reach higher speeds. A proper log features a couple pulls, some highway speed stuff and some stop and go. But I can tell you, you're running rich.

 

[Edecarlo99]

Hey man never posted on Reddit before but always came here for issues I had regarding my 10th gen civic and was having this exact issue you were having. I replaced everything from injectors, o2 sensors, plugs, turbo, maf sensor, etc. I’m running FBO with a w1 turbo at the time with an e30 tune. What finally solved my issue was the spark plug gap on the coils. I’ve never had a high boost and ethanol mixed car before and never had to gap plugs smaller then they came from factory but that’s exactly what I had to do for my Si. The plugs from factory were gapped to .035 so I had to close them to .022-.023 and the car came back to life. Wish I did this sooner but I’ve been having this problem for about 6 months and had to share my experience. Good luck man and hope this helps!

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