1400 idle rpm P0300 p0304

[mek88]

My 2010 Ex35 with 110k miles has SES lite up and showing as p0304. I pulled the cyl 4 plug which looks good with clean white/light grey on electrode’s insulator, and I installed new NGK plug and new original Hanshin coil. I cleaned the driver side throttle body. However the problem persist, and showing p0300 then become p0304 and idle rpm is 1400 when hot. I feel misfire at low rpm, but once i drive it, It smooth out.
I checked and tested the left air intake for leak, install new original MAF. But SES and p0304 still there. What could it be? Appreciate any response and advice. Thank you

[VStar650CL]

The high idle sounds like maybe a vacuum leak. Stream STFT on your scanner and see if it's way positive at idle but drops toward zero when you raise the RPM to around 2K. If so, there's a vacuum leak someplace.

The P0304 is probably a separate issue. The plug insulator should be tan or brown, not white. White means it either isn't firing or it's running lean (not getting enough fuel). Try a new coil, and if that doesn't resolve it, you may have an injector problem on #4.

[mek88]

I rev it up from 1400 all the way to 6000 smoothly, no misfire. Everytime i reset the SES light, the first code always p0300, then later becomes p0304.
Im agree it could be vacuum leak, but i dont know where is it at, and why always cyl4 instead of the whole driver side bank. Thanks

[VStar650CL]

Like I said, I think you have two different problems. Checking STFT to confirm a vacuum leak will save you a lot of potential tail-chasing, it's the most likely cause for the high idle but not the only one.