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Once a year cyl #8 misfire code

Sn8kebitten

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TLDR: TVS coyote has good compression test yet intermittent misfire CEL every couple years and have been told sometimes it just happens with these mods and tuning and to clear it.

So I’ve had my 2017 GT auto since new, always changed oil with full syn Motorcraft. At 29k miles in early 2021, I had a used VMP Gen 2 TVS installed with supporting mods and tuned by PBD, so I switched to 5w50 full syn. I made maybe 6 passes at the local track that year in 2 separate test and tunes, but have not since, and I only baby it on the street.

In late 2022 at 40k miles, the CEL randomly popped up while idling in a parking lot. It was the dreaded cylinder #8 misfire. I reset the CEL to see if it was a fluke because there was no audible or physically felt misfire, but it came back within 500 miles about 5 months later (I rarely drive the car anymore).

So in February 2023 I had the shop who installed the VMP and did the datalogs for PBD do a compression test, and all cylinders were 178-185psi so it seemed healthy. They just chalked it up to dirty plugs from the Octanium I used once at the track in 2021 and replaced the NGK plugs.

No problems for 11 months until March of this year. I had 43k miles on the car, the new plugs having 3k, and the misfire code came back during a grocery trip (never going WOT). I contacted the local shop who said sometimes they see this randomly happen with supercharged cars, and to just clear it and see if it comes back.

I did that and now 1000 miles later, it still hasn’t returned and the car drives perfect like always, but like I said I rarely get on it with how loud it is and easy to blow the tires off. So I guess my question is, is this actually a common thing that randomly happens to custom tuned supercharged coyotes? Or maybe the fact that I only put a few hundred miles a month on the plugs and always baby it, they're getting dirty so I need to change them more often?

I just hate to think this engine could let go any day with only a 79mm pulley and 93 tune so I know it’s not making that much power lol. I’d appreciate your opinions. Thank you
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RAVAGE88

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Is there any pattern of high outside air temp and high engine operating temp when the codes occur?

MB
 

J17GT

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No, I would not consider it a thing with supercharged cars. Have you tried moving the ignition coils around to see if the misfire stays at #8?
 
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Sn8kebitten

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Is there any pattern of high outside air temp and high engine operating temp when the codes occur?

MB
I remember the first time it happened in 2022, it was very hot and humid out and was after I had driven it 4-5 miles in between 4 different places, just shutting it off then cranking up again each time doing errands around town within a span of an hour. The more recent time this year I don’t think it was particularly hot out and it happened on the way back from a single 10 minute drive.

I try to keep an eye on my cylinder head temp and IATs, according to the stock gauge screen anyway. Usually CHT never gets over 200 degrees, but the IATs always seem to be 20 degrees or more above ambient air temp unless I’m cruising on the highway. I always chalked that up to the PMAS intake sucking in engine bay heat
 
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Sn8kebitten

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No, I would not consider it a thing with supercharged cars. Have you tried moving the ignition coils around to see if the misfire stays at #8?
I have not, but that was my plan if the CEL comes back. I plan to inspect the coil boots the next time I change the plugs to see what shape they’re in and probably just replace them
 

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RAVAGE88

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I remember the first time it happened in 2022, it was very hot and humid out and was after I had driven it 4-5 miles in between 4 different places, just shutting it off then cranking up again each time doing errands around town within a span of an hour. The more recent time this year I don’t think it was particularly hot out and it happened on the way back from a single 10 minute drive.

I try to keep an eye on my cylinder head temp and IATs, according to the stock gauge screen anyway. Usually CHT never gets over 200 degrees, but the IATs always seem to be 20 degrees or more above ambient air temp unless I’m cruising on the highway. I always chalked that up to the PMAS intake sucking in engine bay heat
Gotcha, cylinder eight is the hottest running cylinder in the Coyote, so based on the conditions you've described, it could be heat related. You could also try moving the number eight coil to a different cylinder to see if it follows the coil.

MB
 

J17GT

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I have not, but that was my plan if the CEL comes back. I plan to inspect the coil boots the next time I change the plugs to see what shape they’re in and probably just replace them
Why not just flip the coils on 7 and 8 now?
 
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Sn8kebitten

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Why not just flip the coils on 7 and 8 now?
I should. Just haven’t had much time to drive it let alone work on it. After thanksgiving that’s what I’ll do though
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