I've had my GD3 since April 26, 2006. I currently have 177K miles at the moment. During this time, I have had my share of engine misfire incidents. In the past, my Scangauge2 has displayed a variety of thrown ODBII codes, such as a P2138, P2647, P0498, P0304, P0302, and P0451.
Through the years, I have been successful in getting rid of such "misfire," "rough idle," "hesitation," "stutter," "lumpy idle," or whatever else it's been referred to as, by trying the following:

1) you could have bad gas, try adding some fresh top tier gas (maybe try 91 octane)
2) if your plugs are in good shape, make sure the gap is within factory spec .040"-.051" (I personally use a .032" gap)
3) try a shot of injector cleaner (I've had success with Gumout and Chevron)
4) clean the spark plug tops, and use dielectric grease on all electrical contacts (click to see Xcentrick's post)
5) check oil level (especially if a P2647 is thrown)
6) if installed, try disconnecting your "throttle controller" (especially if a P2138 is thrown)

If you throw a P0301, P0302, P0303, and/or a P0304, then try changing spark plug order to see if the code changes.
If the code follows the plug, then I would suspect a plug issue

If the same code is thrown, then that specific cylinder is having as issue. I would try injector cleaner first. If that doesn't work, then I would try changing coilpack order. If the code follows the coilpack, then I would suspect a problem with that specific coilpack. If it doesn't follow the coilpack, then it could be an injector.

If your misfire is not gone by now, then perhaps a valve adjustment is in order.

Please give these ideas a try before you spend big bucks on a coilpack (~$65 ea), a Vtec solenoid (~$150 ea), a fuel injector (~$75 ea), and/or on a valve adjustment (~$150-$300?)

 

Quite a useful post for the DIY'er. Thanks.

 

I have been beginng to notice my fit occasionally "surging" at idle. Especially while cold or in traffic. I have not thrown any codes as of yet. Time to pull the IK22's after 30K and see what they look like. I just had a dealer done valve adjustment less than 5K ago so this is my new check list

 

I pretty much agree with your thoughts on this. I'm not familiar with the other issues, but for a single cylinder misfire (p0301 p0302 p0303 p0304) I would recommend the following DIY troubleshooting order:

1. Mix up the order of the spark plugs and of the coil packs. See if the code follows the coil pack or the plugs, or stays with the cylinder. Although a visual inspection may tell you if your plugs are obviously bad. Check plug gaps when you do this. Also, if coil pack boots are pink-faded red, that's what the factory ones looked like, and that makes a coil pack change more likely. I think the plugs/coil packs being bad is common enough (and this step is free!) so that this is a good place to start.
2. If the misfire follows the plug, replace all four plugs. If the misfire follows the coil packs, I personally would replace all four coil pack boots with the $35 set of four of them from Rockauto. Others recommend the $50 each OEM Hitachi complete coil packs; get them from Rockauto or search for a coupon for Advance auto. I've read on here that sometimes bad coil packs will sometimes be good for a couple weeks after being moved around, although I wonder if that is just an insensitivity to the problem on the part of the engine's CEL system. If one coil pack is bad, it's likely another is on its way out, so replacing all four is advisable.
3. If the misfire recurs only in the same cylinder, the problem is most likely an injector issue or an engine problem. First, I would probably run a bottle of Chevron Techron fuel injector cleaner through the car with your next full tank of gas. I've heard it is helpful to rev your engine up periodically during this tank of gas, as it helps flush out deposits.
4. If that doesn't solve the problem I would compression test the misfiring cylinder along with at least one adjacent cylinder for comparison. If compression is low, that is your problem. I had bad valves on my old engine, and this is what really diagnosed it. Theoretically it could be worn piston rings, but has anyone actually had that happen on a Fit engine? In either case swapping in a lower milage engine may be a better deal than fixing the valves/pistons. If compression is OK, then I would be at the end of my expertise; let the shop figure it out.

As a side note, look at the repair records for these Honda Fits on trudelta dot com. A significant percentage of the engine related repair entries are for individual coil pack replacement. The dealership charges folks $200 to replace one coil pack. Then sometimes they get hit for the same $200 months or a year later when the next one goes.