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Old 01-15-2019, 11:30 PM   #1 (permalink)
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hard to diagnose misfires, part throttle

I've been trying for months to diagnose the following problem:

Misfires consistently in all four cylinders ONLY under medium/part throttle (ie, "normal driving").
Idles perfect. No misfires at idle.
No misfires at Wide Open Throttle.
Does NOT misfire if I drive like my grandma (extremely light throttle).

2008 Civic EX 5-speed R18 with 138,000 miles

No check engine light. No codes. I have a scanner, and I can see all four cylinders are misfiring under medium throttle only. It happens almost every time I use medium throttle. Plugs changed at 108,000 miles (NGK Iridium). I'm open to ideas. I will give results from scanner live data, if you ask me to check something. The challenge here is to not throw parts at it.

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Old 01-16-2019, 04:08 PM   #2 (permalink)
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hard to diagnose misfires, part throttle

I had all 4 cylinders misfire for a while and it turned out to be a bad idler pulley. I bought a cheap one from autozone and it didnít last. It was creating a harmonic that was throwing things off. I changed plugs, did 2 valve adjustments, and a couple other things before I finally had a shop look at it and tell me I should fix that (idler pulley) before I have them diagnose. Worked fine since

I did have a CEL during that time.

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Old 01-16-2019, 11:48 PM   #3 (permalink)
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Was the bad idler pulley making any noise or was it detectable in any other way than the misfires and CEL? Thanks for your input.
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Old 01-16-2019, 11:51 PM   #4 (permalink)
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It wasnít really making any noise but I also never thought of checking it. I think they noticed the belt was split in the middle and then noticed the pulley after that. It was over 2 years ago though


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Old 01-16-2019, 11:55 PM   #5 (permalink)
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hard to diagnose misfires, part throttle

Actually The bracket that itís mounted on was damaged from the wheel shredding it so I think maybe they noticed that.


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Old 01-17-2019, 09:44 PM   #6 (permalink)
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Check your coils. They are run directly off the alternator. So check that also with a multimeter.
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Old 01-20-2019, 12:11 PM   #7 (permalink)
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I remembered today that the failure mode sort of precludes simultaneous failure of all four coils. All four cylinders started misfiring at the same time on a road trip I took about a year ago. And they only misfire between 1500-3000 rpm, and only at partial throttle. They don't misfire at full throttle or idle or light throttle.
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