I know there are probably dozens of threads about misfires here, but every one I've read through is a situation different from mine, and I haven't found them helpful. This will be a long post as I know details are very important when trouble shooting. So, here's my deal. I bought my 2010 Fit last May. It is completely stock and only shows 38,925 miles showing. It runs and drives very well and seems to have good power. Last summer the display claimed about 42 MPG average. Amazing! Last fall the engine light lit and I found codes for cylinder 2 misfire (P0302), cylinder 4 misfire (P0304), random misfire (P0300), and fuel system too rich (P0172).
I assumed the misfires were causing the rich condition, and checked the spark plugs. They looked new, except for one that was slightly dark around the center electrode. I thought maybe one cylinder was running a little rich. I checked the electrode gap, found it was OK, and put the plugs back in. I had no problems for a week or two, then got the same codes again. This time I considered replacing the plugs and coils. Since cylinder 3 had never misfired, I assumed it had a good coil. I decided to check the coils by swapping the cylinder 3 coil with the cylinder 2 coil, which had always misfired. I expected cylinder 3 to then misfire, but it never did, so it appears the coils are fine, at least in cylinders 2 and 3. By then I had a Honda service manual and a scan tool that can show live data. I found the car runs so rich that fuel trim reaches -20%. at idle and drops slightly at higher RPM. Normal fuel trim is about .8 - 1.25 according to Honda. I looked up the procedure for troubleshooting misfire in the manual and it says if there is a code P0172, to trouble shoot the rich condition first.
Honda's procedure for a rich engine is short and simple. It is less than 2 pages. Step 1 is check fuel pressure. It's good at 50 PSI. Step 2 is to check for vacuum leaks at the PCV, EVAP purge valve, brake booster, intake manifold, and throttle body. I checked every vacuum hose I could find and they are all nice and flexible with no cracks or flaws and all connections snug, so I doubt there is a vacuum leak. Next I checked the MAF sensor. Honda says there should be about 4.6 - 5.7 gm/s of airflow at 2500 RPM with the engine warm (coolant temp above 70 degrees C). I tried this test several times and found the airflow went as high as 6.5 gm/s. Sure I had found the problem I ordered one from Ebay. I checked it carefully when it arrived and it's labelled Hitachi and matches what came in my car, so I assume it's genuine. I installed it and found it made no difference. I was about to return the MAF, when I decided to ask my cousin for advice.
My cousin is an auto mechanic with many years experience, and runs his own shop. He seldom works on Hondas but connected his scan tool and found no trouble codes. His tool did register a few misfires when he revved the engine even though it seemed to be running well. His advice was to leave the MAF alone for now and replace spark plugs. He said Irridium plugs can look good but still misfire. He also suggested checking for a leak at the EVAP canister purge valve, as a leaky valve could allow fuel fumes into the engine from the fuel tank, making it run rich. Yesterday was unusually warm for Minnesota, with highs around 50 -60 degrees F. when I got back from my cousin's shop. I ran the MAF sensor test again, and this time it passed ! (I think) My cousin said they can be difficult to test as airflow values vary with air temperature, humidity, and other factors. So, Now what? The next step in the service manual is to adjust the valves.
Here are my questions. Would the valve adjustment cause rich running and a mild misfire ? Apparently Honda thinks it will, but if so, how? And would it be a problem in a car with only 38925 miles? Also, could the plugs be bad with such low mileage even though they look normal ? I believe they're supposed to last 100,000 miles. Some threads have suggested using only Denso plugs and setting the electrode gap at .044 inch. I don't recall which plugs I have, but I think they're NGK and are original to the car. One thread suggested cleaning the ignition coils where the springs are. Does that make sense? Also, how likely is an EVAP purge valve problem, and is there an easy test for it ? As always, I appreciate any and all advice.

 

So far I have a few views but no advice. That's a little disappointing. I've watched a few You Tube videos since I posted this so I'm adding what I've learned. According to You Tube and some other threads, it appears tight valves with too little clearance can cause misfire, rough running, and cold start issues. One valve adjustment video suggested checking the valves every 30,000 miles! Is that really necessary ? I expected valve adjustments to be done around 60,000 miles or more. I cannot find a maintenance schedule for my car, so I have no idea when valve adjusting is typically due, or how long spark plugs are supposed to last.
Another thing I learned is the EVAP system can check itself for leaks, and will trigger a trouble code if it finds one. Since I have no codes, I suppose it's safe to assume the EVAP is fine. So that leaves two reasonable things to try. Those are valve adjustment, and spark plug replacement. I suppose I'll do both while the car is taken apart.
My next question is where should I buy the parts? It looks like I need a valve cover gasket, four intake passage gaskets, a throttle body gasket, and the spark plugs. I should also get a blue cowl trim clip, a white trim clip, and the hood insulation clips. It would be nice to find one source for everything. Any suggestions? Also what spark plugs? I see Honda used NGK and Denso. Some really prefer the Denso plugs. Also what electrode gap? Some suggest .044"is essential. And how tight to put them? Some say 20 foot pounds, others more like 14?