2008 Fit, 106,000 miles. I did the valve adjustment myself (and I don't feel comfortable that I have the skill for that job). Replaced coil packs and plugs about 10,000 miles and 18 months ago. It is my wife's car. I just changed CV joints and tranny fluid. Now I am noticing mis-firing. autozone's diag computer said it is missing on 3 and 4. The missing is under heavy acceleration above 4k rpm. Closer to 5K.

Could this be the valve adjust needs done better? Can a mechanic with a better computer give me better info?

 

Now I haven't dealt with this thankfully so take it with a grain of salt. I think it could be the valves, but I'd check spark and fuel first. I know you changed the coils recently but they could be defective. Also maybe a compression test could show if the valves aren't closing fully for example.

 

Loose spark plugs!

 

I have tightened all the plugs, and still misfiring reliably between 4 and 5k rpm. I am questioning valves since it is a variable valve timing, and it is missing only at the upper register of RPM. Any more thoughts from the community?

I also have seafoam fuel additive in this tank of gas, which I am also questioning as a possible culprit. I cannot say for sure that the missing started after the additive, but I cannot deny it either.

 

So, I have also found that when the engine is cold, it runs fine. The engine revs right up to the redline. If anyone has any suggestions, I would appreciate it.

 

I'm at 101k with the exact same issue. mine is getting plugs, coils, and a valve adjustment tomorrow. I've seen thread after thread here and on reddit about this issue right at 100k and that is always the solution. my theory is it actually comes down to the coils but it's simple enough to just do the valves and replace the plugs and coils on reassymbly anyway so...

 

Will pull the valve cover and check the valve clearances
cylinder 4 is closest to transmission just in case

 

Valve adjustment is exactly about getting the right tolerances when the engine is hot. If the valves were adjusted when the engine was still warm, the tolerances may not be quite right. Your cold engine runs well seems to suggest the valves are too tight; when the engine warms and the clearances reduce, the engine would run poorly. Typically, intake tends to be too wide and and exhaust too tight. You might try increasing the exhaust to the upper end of the manual tolerance.

a real easy check would be to remove the coils and put them on 1 and 2. If you still get misfires on 3 and 4 then you know for sure it’s not the coils. If that doesn’t change anything I would do the same with the spark plugs to eliminate that possibility. Swapping the coils abd then the plugs would take less than a half hour so it’s an easy check compared to removing the valve cover. I would try that first. If it’s still 3 and 4 then check the valves again.