2002 Accord EX V6 has lately seemed like it was just a little off somehow, but nothing I could put my finger on. Maybe just a little more sluggish than usual. As of a couple of days ago, it started idling *very* rough, the MIL flashes continuusly, and the TCS light stays on. It does smooth out with some throttle, but is pretty much undriveable. I have the FSM, but don't have the V6 addendum, so it's not real helpful here. I checked for codes, found 2-3 cylinder misfire codes, the "random misfire" code (P0300), as well as P1399. I've checked the coils and plugs, they are all visually fine. I pulled the intake cover and EGR valve to make sure they're not blocked, they were clean. I've reset codes and shifted all the coils by one, but the misfire codes don't necessarily move with the coils. Nothing noticeable happened, it was driving fine and then the next time I started the car it was misfiring and shaking like crazy. Any suggestions? The I4 FSM says to check the distributor housing when it's misfiring at idle, but that doesn't apply to the coil-on-plug V6, right?

EDIT: I forgot to mention that some research pointed to the need for a valve adjustment for code P1399, which I don't know when was last done, if ever, on this car (235K miles). The I4 FSM mentioned valve clearance for "Random misfire at high RPM and load, or under random conditions". Mine is consistently misfiring at idle, so I haven't gone to the trouble of trying to adjust the valves yet.

 

This isn't really an "Internal" engine issue, so I'll move this over to General Tech where more people will probably see it.

True, your car doesn't have a distributor so you can't check that. But have you pulled the spark plugs to check what they look like? Marks from arcing across the surface of the porcelain insulator? Burned electrodes? Gap OK?
When you checked the coils, did you see any evidence of arcing around the end of the coils?

Why not check the valve lash. It's not that hard to do even if it turns out to be OK.

 

I doubt the valves are the issue unless you found a valve way off spec. I found the valve lash tighter than the factory specs when I tested mine and suspect they were like that from the factory.

When you moved around the coils, what codes appeared?

Any chance you filled up the car with gas before this problem happened?

Can your scanner read live engine data like fuel trims or misfire counters?

 

After moving the coils, I still got misfire codes and P1399, but the misfire codes didn't follow the coils. When I went for a short test drive after I had moved the coils and cleaned the EGR passages, I had two misfire codes before I left and three when I got back. I did fill up with gas not too long before this started happening, but it was at a gas station near my house that I've used regularly for years. I put a can of Seafoam in the previous tank, which I did think was a strange coincidence, but I've never had Seafoam cause a problem before.

My scanner does read live data, but most of the stuff that I work on is older, carbed, and non-OBD II, so I'm not familiar with troubleshooting that. What should I be looking for?

 

I did some more digging and I found that my short-term fuel trim is pretty consistently running between +8 and +12, which if I understand correctly is too high, but not insanely so. Is that high enough to cause rough running? I only saw one bank reported, even though it's a V-6; I don't know if that's expected or a limitation of my scanner. Longterm fuel trim is at 0.0, but I think that may have gotten reset the last time I cleared the codes. I also found a couple of splits in my air intake hose (between the filter box and the throttle body), could that be causing this?

 

+ fuel trim means the PCM is opening the fuel injectors longer than perfect conditions.

Monitor your long and short term fuel trims at the same time, because the sum of them is the total fuel trim. Let us know the total fuel trim at idle, then the total fuel trim at 2000 rpm. That can help direct your next step in troubleshooting the problem.

 

Update: I replaced the air inlet tube, which had split nearly in half in a couple of spots. It helped significantly, but didn't completely fix the problem; it runs a lot smoother than it was, but still rough and I get a flashing MIL and TCS disabled within a couple of minutes. Short term fuel trim is now running about +7 to +9 at idle, longterm is still 0.0. At 2,000 RPM it was in the +2 to +3 range short, 0.0 long. I reset the codes and cylinder five misfire came back pretty quickly, but no others (so far). To reiterate, when I pulled all of the plugs and coils a few days ago there were no signs of arcing or plug damage on any of them, but I'm going to focus on number five in particular now since it's the only one that came back.

EDIT: Moved the coil from five to six and cylinder six started reporting a misfire. I pulled the connector from six at that point and it made no difference in the way it was running, so I'm convinced now that that coil is bad. However, cylinder three also started reporting a misfire. I swapped the coil and plug and three still reported a misfire, but when I pulled the connector from three it started running much worse. Is it possible a bad coil on six could throw three off enough to report a misfire?

 

The firing order is 1-4-2-5-3-6, so maybe 6 is throwing off 3? Since you did the power balance by unplugging coils, replace the bad coil currently on 6 and retest.

 

I've been away for a couple of weeks, but just to followup: New coil on 6 only, much improved performance and no codes since.

 

Typical, one bad coil will almost always report as multiple misfires.