I'm sharing my recent engine troubles here so others can benefit from what I learned.
Our '99 Ody LX began running very rough, and the CEL came on. Codes reported were: P0300 (Random Misfire), P0301 through P0306 (Cylinder 1-6 all misfiring). There was also a code P1399, which is a temporary DTC.
In my Honda Factory shop manual the troubleshooting tree suggests looking at the following, in no particular order, although, depending under what driving conditions the misfiring occurs, the list is narrowed:
(1) Check fuel pressure.
(2) Check cylinders for low compression.
(3) Low quality fuel.
(4) Correct EGR valve function.
(5) Malfunction in the VTEC system.
(6) Correct valve clearance.
I was having misfiring under moderate acceleration, as well at WOT...the CEL will actually blink when the engine is misfiring. I have had a lot of experience with vehicle fuel systems getting plugged at the fuel inlet screen attached to the fuel pump inside the fuel tank, so I naturally dove right into diagnosing the fuel system. After finding the correct M12 x 1.00 fuel pressure adapter (can be purchased from A&E Hand Tools for $37.32 at www.aeincorporated.com p/n 262554), I measured the fuel pressure at idle, and while driving...all was right in spec. I then looked into the EGR valve, pulled it off and cleaned out all ports and the valve itself very thoroughly, even pulling the manifold off to get to the manifold EGR port. The car still ran poorly after cleaning the EGR system and checking that the EGR valve worked properly. I then pulled all the spark plugs to do a compression test...readings were right on the money. I then removed the manifold AGAIN and pulled all the fuel injectors (don't know why I didn't do it the first time)...I cleaned them in mineral spirits in an ultrasonic bath for 30 minutes then put it all back together. Still running rough. I ended up replacing all the spark plugs after doing the compression test....the #5 spark plug (front bank, middle cylinder) was black and oily, while the other 5 were nice and clean. This was the clue needed. After disconnecting the #5 coil, no drop in rpm was noted. Any of the other 5 coils caused a drop in rpm when disconnected...I swapped cylinder 5 and 6's coils and the problem followed the coil when doing the cylinder drop test again. Alas, a new $67 coil on the #5 cylinder and the vehicle runs like its old self again!
Here's the learnings to pass along:
(1) Utilize your 'basics' when troubleshooting. Fuel, spark, and air. Use DATA (i.e. fuel pressure measurements, compression readings, voltages, etc) to determine and assess the health of an engine subcomponent. A mechanic's stethoscope, or a long screwdriver in my case, works great at listening to fuel injectors for proper solenoid operation. Note the factory shop manual did not point to the ignition coils at all, so don't get tunnel-visioned on what the computer and factory shop manual is telling you.
(2) Spark plug inspection will tell you an incredible amount of information about what is going on. Most Chilton or Haynes manuals have a section that explains various spark plug fouling and what can cause it. This was the smoking gun in my case.
(3) Work methodically from one subassembly to the next and elimate each subsystem in sequence as a potential cause before moving on.
(4) If you're going to remove the manifold, clean the EGR port (I used carb cleaner and a 3/4" diameter x 10" long engine brush out of a kit I got from Eastwood.com for $6.99 p/n 46035) as well as the fuel injectors. I sent out my 1992 Toyota Pickup's fuel injectors 3 years ago to Cruzin' Performance in Michigan for *REAL* cleaning. They disassemble, bead blast, ultrasonically clean with special fuel injector cleaner (all while driving the solenoids at a safe duty cycle), reassemble with new pintle caps and o-rings, then measure the flowrate and visually check the spray pattern before shipping back to you. They give you a before and after cleaning flow test. If your injector set is not within 5 or 10% balance (can't remember which value it is) for the whole set on the flowrate, they create a matched, balanced set by swapping your clean injector with an identical one they have in stock. Their website is www.cruzinperformance.com and I spent $84 + shipping for my 1992 Toyota Pickup injector set (6 + 1 cold start injector). After reinstallation, the pickup's idle was smooth as glass...just like a brand new truck. Also, the turnaround on the injector cleaning was only 3 days!!
(5) Invest in basic troubleshooting tools like a factory shop manual, compression tester, fuel pressure tester (and M12 x 1.00 Honda pulse-damper adapter), timing light, digital volt meter, etc, if you don't already have them...and learn how to use them.
(6) Lastly, these forums are good information sources, but make sure you double-check the accuracy of any and all information (particularly numerical information) you get on the internet...especially from online forums.
Incidentally, the Honda Odyssey factory shop manual had no troubleshooting of the ignition coils anywhere in the book, nor did it even have resistance spec measurements for the ignition coils.
Hope this helps someone out there troubleshoot their Ody. I noticed there are a lot of posts on cylinder misfiring, so I thought I'd be as detailed as possible to help those out who are or will be in the same boat I was in yesterday.