2002 EX (USA) with 250K miles on it. It's misfiring on 3 and every now and then 1 or 2 but have not seen all 3 at once. It seems that it happens after it's warm and more often when accelerating gently from cruising, but can happen at about any time. Seems to last for about 3-10 seconds at a time though I try to get it out of it once it starts. Another odd thing is an issue that I had a few years ago that disappeared on its own has reappeared twice now, which is the speedometer will swing quickly down and up (maybe about 20MPH from actual) and then go back to normal.

I have swapped plugs (OEM Denso with about 15K), coils (2 had been replaced 15K ago) and injectors (all original I believe) and the issue (P030X codes) doesn't seem to move. Am I correct in thinking anything else dealing with fuel or spark would affect ALL cylinders? Thus, I think I am down to compression and valve adjustment. Anything else I should be looking at?

 

Valve adjustment first... then a compression test. You are on the right track.

 

It's intermittent, so doing a compression test and valve adjustment is not going to solve your problem. Did you use a torque wrench when you installed the plugs? Why didn't you use the ngk plugs? Is there oil down in there when you pull off a coil? If you go to the pull it your self junk yard you could grab 4 oem coils, and a gauge cluster. Id guess gauge cluster over vss, but you could take the cluster back and get a vss. You could take the coils back and get a head unit or whatever. My civic came with some new platinum plugs in it, and started having a misfire. I found a spark plug extremely loose and another kinda loose. The really loose plug actually began to destroy itself but sometimes managed to fire. It rattled when I shook it.

 

Yeah, the intermittent nature is the main problem, just like trying to debug software issues that aren't repeatable. Well, being intermittent AND the fact it is on more than one but not all cylinders. Interestingly, the service manual indicates that for "any combination" of the misfire codes, the main thing to check is fuel pressure and then depending on what conditions it occurs to also check for: Low compression, Malfunction in the VTEC system and correct valve clearance. I just don't see how fuel pressure or VTEC would not affect all cylinders???
Yes
Because both Denso and NGK are well-regarded OEM spec, and I didn't install this particular set originally. I intend to go back to the NGK platinum that I had before.
Yes, in cylinders 1, 2, and 4 so not the cylinder that is having the main misfire. I cleaned it out before putting them back, of course, and will replace those tube seals when I verify the valve adjustment.
If there is no way it is related or will cause an issue, I'll just ignore it like I did last time (over 6 years ago) since it happens rarely. The cluster does pass its self-test.

 

Compression doesn't seem too bad for such an old engine.
195
190
180
195

Once I get the parts, I'll adjust the valves. Trying to decide if I should tackle replacing the valve seals as well as I suspect I burn some oil because they are worn.

 

I haven't replaced valve cover gasket or any seals etc. I do have a little oil around but it's not too bad, and I haven't had any misfire issues in the last couple years. I'm sure there would be some oil down in there if I pulled a coil off, but things are working fine now so i'm not messing with it. I put new 0-20 mobil 1 high mileage in each spring, which is 9-10k miles. If I junked the car tomorrow for zero dollars I would be at around 76 bucks a month cost of ownership. Almost 200k on it and everything is original except for locks, stereo, tires etc.

 

May be good idea to pull coils and also the injectors (or whole rail) off a low mileage car and swap that stuff over. You can always return it if nothing improves.