'99 Accord Misfire codes P1399, P0300-P0304

I have read through all of the previous posts regarding these codes and tried various advice to no avail.

Background: 1999 Honda Accord EX, stock, 125,000 miles, fairly good maintenance, but not perfect. Driving without any noticeable drop off in performance until the event described below.

Just this past Sunday my engine started running really rough and the check engine light started flashing. The OBD II codes are P0300-P0304 and P1399 (P0300- random / multiple misfire, P030x = Cylinder x misfiring, P1399 = Manufactures control Ignition system OR Misfire). Per advice found on this forum, I pulled each of the plugs to see if it was one of the ignition coils, but they all seem to be ok (made the engine run worse when unplugged). The engine runs fine at all RPMs in neutral, but under load it runs like garbage until about 2800 RPM, then suddenly jumps to 3000 and runs fine. When the transmission shifts and the RPMs drop below 3000, it again runs like garbage until it gets up to about 2800 RPM again.

Yesterday I checked the fuel trim numbers (again, per advice on this forum) with the following findings:
1. I/M Status: Catalyst, Evap Sys, O2 Sensor, EGR Sys - Not Ready
2. Short term fuel trim was all over the board, but mostly in the small numbers +/- 5% or so. It seemed to jump to around +20% at the worst of the rumbling, just before jumping to running smoothly again.

Some of my thoughts, based largely on only a rudimentary understanding of how the fuel system works: It is probably getting too little fuel or too little air, so it could be the air cleaner and fuel filter. It seems that it wouldn't be the injector since all the cylinders are misfiring. (However, if one cylinder was misfiring, could it cause the others to misfire?) It could possibly be a bad O2 sensor messing up the computer's calculation of fuel needs.

Additional background info: I was driving in the mountains in WV all weekend, the weather was warm and wet, the car was fairly empty and I wasn't hauling anything, my passenger was being somewhat annoying, and Sam Cooke was on the stereo.

Any help you can provide will be greatly appreciated.

Cheers,
Ash

Diagnose the P0300 random cylinder misfire and the P0304 cylinder misfire how long have the spark plugs and the spark plug wires been on the vehicle ?

Unless they get change at the 100,000 mile maintenance, never. I'd have to check my maintenance records to be sure.
-Ashley

Ok that sounds pretty good

Hi, did you ever find the solution to this? My accord is giving me P0302 and P1399 codes. I'd appreciate any help.

Sam cooke is always good for a roadtrip

On most Honda's when you get a misfire happening across the whole engine like that there are a few likely issues. the ignition coil if it's a single coil style or the connector at the distributor. They alsp have some issues with egr's and plugged ports which can mess with the fuel trim numbers. It's important to check the fuel pressure and if possible a timing offset spec from the scanner whenever you're dealing with misfires on any vehicles Honda or not and determine correct pressure and flow whenever possible. Was your reading of +20 on the fuel trims taken at idle? How does the 02 sensor appear to be switching. After it smoothed out what did the fuel trim number do?

Turned out to be the spark plugs. Thanks for the tips. I checked everything before checking the plugs. Next time I'll start with the easiest and cheapest, whether or not I think it is likely.
I assumed that since all cylinders were throwing misfire codes that it couldn't be the plugs. Apparently, though, that kind of thing happens fairly regularly. A friend explained that if all of the plugs are pretty close to the end of their life, one can go, creating higher load on the others, which in turn makes them misfire as well. I don't know that I buy that entirely, but changing the plugs fixed it. Live and learn.
Thanks again for everyone's suggestions and tips. I learned a lot about my engine during this exercise.

Congratulations!

I wasn't so lucky. I changed the sparkplugs and the problem still persisted. Ended up bringing it to my mechanic. He did a leakdown test and cylinder #2 was bad. Diagnosis was burnt valve to the tune of $1400.

Mechanic said Honda specifies a valve adjustment every 105k miles or when it gets noisy. Apparently the valves tighten up over time and will not make noise unless they were loosening up. So lesson learned is to adjust the valves more often on these Honda motors.

That's good info to have. I just adjusted the valves on my car a few months ago. Turns out that once you figure out what you are doing, it's not terribly difficult to do.