2004 Honda Accord 3.0 misfire at idle and p0300 and 1-6 cyl mifires

I have an 04 Honda Accord 3.0 v6 with a rough idle and codes p0300,p0301,p0302,p0303,p0304,p0305,and p0306. My initial visual inspection didnt reveal much except that I could tell that this car has been "repaired" a few times! I assumed that the cylinder 1-6 misfires were caused by someone unplugging each coil trying to pinpoint that dead misfire I found on cylinder 5.
Scan data showed cylinder 5 misfired the most and confirmed a bad ignition coil.I reccommended one coil and six spark plugs. Changed 5 out of 6 plugs because cylinder one plug was not budging! So after one coil and 5 spark plugs replaced I still have a rough idle and slight misfires here and there on any given cylinder. After doing a visual inspection and cleaning the thermostat housing ground and the throttle body I still have a rough idle. Also the first start right after the throttle body cleaning, all of the misfire codes came back for cylinders 1-6 and a p0300 as well, but cylinder 5 was no longer a dead misfire because the coil fixed that problem. This seems like an experience related answer and I have a lot of clues as to what could be wrong( valve adjustment,slack timing belt, low cylinder compression, cylinder leakage, etc etc etc) but I need some direction before I spend a week on this problem that seems to be a possible simple fix. Please share any info or experience that you guys and gals may have on this subject. Thank you!

With Honda engines valve clearances are always a suspect where general rough running or a multi-cylinder misfire is present

Andy is right on with his suspicions of a valve clearance issue. How many miles on this Accord? If it's over 100K and no adjustment has been done (or was done poorly), then it's time. It's really not a bad job, just kinda rough on your knees while leaning over the engine to do the back bank. :lol: Clean the EGR passages in the intake and head at the same time to prevent future EGR flow codes.

An easy check you can do is observing the MAP sensor voltage at hot idle with no loads. Should be at .9V or lower. If not, that suggests either a vacuum leak, or a valve clearance issue.

I've never seen a slack timing belt cause this kind of symptom, but I suppose it's possible. I've found that if the belt is THAT slack, or skipped, then it results in a severe idle problem or a no start. A relative compression test could answer a lot of base mechanical questions. Keep in mind that tight exhaust valves can change peak compression numbers if they get tight enough.

ive done a few tsbs on this engine it included reflashing the pcm/crank shaft relearn. i chased my tail a bit on the first one i seen


www.revbase.com/BBBMotor/TSb/DownloadPdf?id=128969

I didnt see a lean code but make sure its not a winter blend fuel vapor lock. there is a tsb on it and I just saw one last week.
Ideally if this is the problem it will show its ugly face after a hot soak of about 5-10 minutes.

No lean codes and as soon as it starts anytime hot or cold it trembles and runs rough. I was thinking bad gas as a possibility, but also was thinking valve adjustment was needed, but I have never had to do one on a Honda before. The customer said her husband drove it to work and it started to misfire but he got it scanned at Autozone and drove it home. She assured me that no one has ever worked on it since she has had it, but then she said her husband did the brakes on it, so I am not feeling like I have the whole story. That being said I took one last shot at it and then I gave her her money back( even though I did fix her dead miss on cylinder 5 with a new coil) because I didn't communicate clearly that the dead coil was first priority then we can go from there after some fresh spark plugs. Anyways I still would like to know what the issue was. Thanks for the responsiveness to my post.