"Misfires and You" by Derek Robinson
a.k.a. How to diagnose your P0300 (301, 302, 303, 304) codes.
Plugs, coils, injectors, valve lash, head gasket, ringland. Those, in that order, are the most probable causes of misfire conditions on L15 engines.
Plugs are pretty self explanatory; I've seen fouled plugs, broken porcelain (where it will be broken and slide up and down the electrode when shaken), etc. If the misfires occur only under high load at elevated boost pressure or in colder weather, the default spark plug gap may be too wide. In that instance, reducing the plug gap down to 0.020-0.025" will solve the problem.
Coils; self explanatory here again. Coils fail and you can diagnose this by moving the coil to another cylinder to see if it moves with it.
Injector; this is a trickier one. Injectors can fail intermittently or completely, and it's very difficult to diagnose. I've seen failing injectors cause only slight misfire conditions to occur at high engine speeds/loads, and I've also seen them stick completely shut (shutting one cylinder down completely) and fail open (hydrolocking the engine). Injector failure is a very, very common problem that many do not talk about, and is often misdiagnosed by dealers as spark plug/coil problems first. Ignoring a failing injector can result in many other problems, like the ones below.
Valve lash; some engines have a tendency to tighten up valve lash as time goes on (the valve actually sinks deeper into the head from wear on the valve seat). This leads to eventual loss of compression as the valve stays open slightly all the time. This gets worse as the temperature of the engine rises.
Head gasket; now that the L15 is a few years old, and people have used and abused them for thousands and thousands of miles, I've seen some head gasket leaks on stock cars, lightly modified cars, and heavier modified cars. It seems more prevalent in cars that have been run hot in the past, and/or load the car down hard when making a ton of low end torque (something many stock turbo ethanol users seem to like to do). The surface finish on the L15 blocks/heads is pretty laughably poor, and the head bolts fatigue, allowing combustion gas to be expelled into the cooling system. This creates an irrigation path for coolant and combustion gas back and forth. When cold, coolant will run through that path and puddle in the bore, where it will be burnt off on startup, and the car will sound like a Subaru on cold startup for a few seconds until the coolant is burnt (which you can see significant misfire counts occur at this time). This will be accompanied, of course, with coolant consumption. If left unchecked, this will get worse and worse, potentially damaging other components. The fix for this is, of course, gasket replacement (new OEM Honda), resurface the cylinder head to a much finer surface roughness quality by a good machinist, and using ARP 271-4701 head studs (yes, the ones listed for a GSXR 1300).
Ringland failure; this is by far the least common failure on the L15. The piston's second ring expands from heat to the point of butting the ends of the ring together, deforming it and putting upward or downward pressure on the ringland that carries it. This will be accompanied by compression loss, of course, and require replacement of the piston
Source - Derek Robinson (Facebook)