I have a 2000 Honda Accord LX.. 4 cyl. I was driving to work and check engine light started blinking.. blinked for about 15 seconds.. then just stayed on. I pulled codes and got a P0304 code, cylinder four misfire. So... I changed my spark plug/wires/distributor cap and rotor, then cleared the codes. All looked very, very worn. After changing the plug/wires/cap/rotor, I test drove it. It idles fine... very nice actually. When driving down side streets, all seems well. Then I took car on highway... you could feel the engine miss.. or studder just a little bit, and just for a few seconds, then it seemed to run smooth again. The check engine light would then blink for about 15 seconds, then go back out. Whenever I *pushed* the car (RPMs go up) this would kind of happen. The check engine light would blink, then go back out.I pulled the codes.. and the only code I got was a P1399 code. There is very little information that I can find on this. Anyone have any ideas? Any help is very much appreciated.** Car has 185,000 miles on it. I've had the car 4 years now and it had 123,000 miles on it when I got it. Never had any real issues with it until now.

You changed all 4 plugs? Did you use NGK plugs? The correct ones? Are they tightened correctly? Did you damage any when you put them in?

Did you rout the wires correctly and not have them overlapping each other? Are they the plugs and the distributor firmly?

Cap and rotor on correctly?

You could have a bad coil...a bad injector...Maybe a bad ICM...but I would check the work that you did and make sure it's all done right and no mistakes were made.

Thanks for responding. I meant to include that. Yes.. I used NGK plugs, the exact kind that the owners manual calls for (forget model number at the moment) and it is also what was in car previously. I also used NGK spark plug wires. The wires are not overlapping. They're following the same routes/guides as the old wires did. I believe everything is plugged in firmly, but yes... I will recheck all connections tomorrow.
I don't believe I damaged the plugs while putting them in... I tried to be careful. What is the correct way to tighten? I tighten them till the were snug... then maybe another 1/4 turn.

That sounds about right. Like I said, i would just double check.

We have an expert here that is real good with Hondas. He should be along some time tomorrow to give you an idea.

P1399 is likely a random engine misfire, usually caused by fluctuating engine rpm. The ECM picks up on this through the Crankshaft Position Sensor. First, I would replace the Ignition Control Module (ICM) and coil. These two components are the heart and sole of your car's ignition system. The intense heat inside the distributor is really hard on these components. I recommend doing this every 125,000 miles or 10 years, whichever comes first. Be sure to apply silicone heat transfer compound to the back of the new ICM or it won't last long. Second, I would get the valves adjusted, when the engine is "stone" cold. Third, I would focus on the EGR system--passages may be blocked or the EGR may need replacing. Start using full synthetic 5W-30 engine oil. This will help the EGR system to stay cleaner, besides a host of other benefits. Change the oil and filter every 6,000 miles or 6 months, whichever comes first.

Thanks for responding. So you're saying that I should replace ICM and coil... should I just replace the whole distributor? About the valves getting adjusted. I've never done this before. I've watched some Youtube videos.. and doesn't look that hard, but I'm still not convinced I feel comfortable with it. How much would something like that cost to have done?
I have read a few things about the EGR system, so that was the only thought I've had so far to check. I've used full synthetic oil (Mobile 1) since I've owned the car, and I use a Bosch oil filter. Change about every 6,000-7,000 miles.
Thanks again for your input.

Yes, I would only replace the ICM and coil, at this time. Aftermarket ICMs and coils have been fine. If you decide to replace the distributor, only install a genuine Honda distributor housing--all others are problematic. Scribe the distributor's mounting bracket with an index mark, prior to removal. I don't recommend you adjust the valves yourself. It's an acquired fine tuning skill, where you can crack the exhaust valves if they are adjusted too tight. Your Honda dealer will likely do it for around $75. When you replace the ICM and coil, remove the coil first. This will provide much better access to the ICM. Be sure to use a precision tip Philips #2 screwdriver, so you do not bugger the heads.

Since you have been using Mobil 1, the valves and EGR system are not likely the problem. Do you hear any excessive valve noise?

So you're saying that getting an ICM and coil from like AutoZone will be fine, as long as I still have original Honda housing? What about all the other internal sensors/components within the distributor? Could they be the issue?
And no.. I don't seem to hear excessive valve noise.

AutoZone ICMs and coils are fine, as long as they are going into a genuine Honda distributor housing. If it turns out your current distributor housing is bad, you can install your new AutoZone ICM and coil in a new Honda distributor housing and you will be money ahead.

Ok... I did not change the ICM or the ignition coil because I took the ICM to AutoZone and had them test it... it tested good. I check the coil with a muli-meter (ohm test) and the coil feel within tolerances. As I was getting the ICM tested, AutoZone looked up the codes and one thing that was listed was EGR passages. I know you said that since I use Mobile 1 that I should not have no build up.. but I bought the car with 125,000 miles on it, and wasn't sure what the previous owner used.. so I took off the upper intake manifold off and checked the passage.. it was really not that bad. I cleaned anyhow. I also changed the EGR valve to test that as well. Both of these did not fix the issue. Still get the P1399 code. Not sure where to go from here. I was just thinking if I should change the spark plug gaps. From everything I read, if I get the plugs that came with the car, which were NGK PZFR5F-11.. which I did.. then I shouldn't have to worry about setting the gap on them. That they come pre-gapped at .044". I checked before I put them in and that's what they were. However.. sitting here racking my brain on what would be causing this issue.. I was reading the repair manual.. and it says the gap for the spark plugs should be .039-.043. Well.. these NGK's are pre gapped for .044". Would this be an issue?? I would think so.. but these are the plugs that came with the car and from everything I read it says you shouldn't try and change the gap.. that you could break them. So I'm stumped.
I just don't get it because the car seems like it runs fine. I keep the code scanner hooked up while driving and keep checking while driving.. sometimes the p1399 shows up without the check engine light blinking.. and sometimes it blinks... but in either case.. you don't really feel any missing. If I didn't have a code scanner hooked up.. sometimes you might not even know there was an issue.

I can't tell you how many ICMs I've seen that checked out fine on AutoZone's Wells Machine that were, in fact, bad. Ditto for testing the resistance in coils. Many coils start to intermittently break down under load (heat). I still think the solution is to replace both the ICM and coil with brand new components. .044" spark plug gap is fine.

OK.. so.. since you suggested I replaced the ICM and/or coil.. I figured I'd just replace the distributor. I replaced the distributor and it had no effect on the issue... still got the P1399 error and can feel a slight miss. Any suggestions on what next steps are. Thanks.

Was it a genuine Honda complete distributor?

Yes...
So.. I haven't been driving the car to work... just starting up in drive way.. and maybe short trips around the block just to check if something I tried worked or not. Today.. I figured I'd drive it to work to see how it would act under normal conditions.
When I started it.. it idled slightly rough.. not enough to make me abort my decision to drive it.. then after that I had a short trip to the highway.. seemed to run fine... might have felt a slight miss here or there (not sure if at this point I was imagining) but no error codes or CEL. Got on highway and figured this might be where the issue really comes to light. However... I drove about 25 highway miles at about 75-80MHP.. mostly about 75. And it was running very well.. no misses that I could feel, no codes popping up (have scanner attached) and no CEL. At this point I'm kind of hopefully thinking it's just something that needed to burn off or something. However.. once back on city stop and go roads (for about 5 miles) you could really feel the hesitation and missing going on when slowly accelerating from a red light. Then.. check engine light pops on. Then a little further down the road.. when again, accelerating from red light... felt it miss a little more.. then the light blinked and stayed on. At no time did I feel the car was just going to die though. Code when I got to work.. P1399.
Does any of what I've described help at all? Thanks.

Be sure and clear all old codes. I would also check the timing with a timing light. Make sure there are no air leaks; e.g., intake manifold gasket, PCV system, and hoses going to the plenum and MAP Sensor. If the problem persists, check each cylinder's compression with a gauge.

I did check compression.. all cylinders seem to be about in the 135 range.. they were all pretty even. The upper intake manifold had a metal gasket when I took that off to check EGR passage.. so l left that on. Hoses all look good.. didn't check MAP sensor though.

With the engine running, take a spray bottle and spray water around the intake manifold--listen for any change in the sound of the engine. Finally, check the Short-Term Fuel Trim and the Long-Term Fuel Trim readings with your scan tool. That could shed some light on the problem. Those are about all of the ideas I can think of.

Have you had the valves adjusted? P1399 Honda Random Cylinder Misfire Detected OBDII Engine Light Trouble Code | Engine-Codes.com

I would pull the spark plugs also and look at them, see if maybe you can see one that isn't firing as it should. Keep this in mind too...I have seen defective plugs right out of the box (not common but it happens) and I have also seen people install them and crack the insulator, hairline crack, when putting them in. This could cause an intermittent miss. With all that you've already spent, it may just be worth it to have the valves adjusted and buy a whole new set of plugs and put them in. I know you don't want to keep throwing money at this but it may just be worth trying another set of plugs.

A bad batch of gas will also trigger the symptoms you have detailed, so have you treated your gas tank, or bought gas from a different station than normal?

Wanted to close the loop on what the issue was. It ended up the fuel injectors were the issue. I replaced all four. I'm sure I could have done maybe just cylinder 4, but did all four. Car has been running like a top since. Thanks to everyone for their help. Especially to TxGreaseMonkey for taking the time to help.