(Issue is resolved with new set of ignition coils)
Hi all...Have been working on this issue for a few weeks and scratching my head..... 2k Accord LX\V6 with 140k miles... I have searched and read many posting on the internet.

It started about 3 weeks ago when family drove it to grocery shopping. They came home without any complaint. Then son drove the car and came right back telling me it threw CEL and misses. I immediately test drove it. It is intermittent. Sometimes, it will be smooth for a whole 10-15 mins drive then misses and it will smooth again. It is really difficult to find a pattern, not cold or warm related. I pulled the codes and I've got the random miss fire, cyl 5,2,3 miss fire. Sometimes, the code can be only one cyl. The problem is most noticeable at idling, it can be smooth then randomly buckling with rpm dips about 100 then come right back to 700 rpm. During cruising, it still misses but it barely noticeable (but still noticeable). Code has been cleared many times after every possible fix and they all coming back as misfire code, either a single cylinder or random miss fire (more than one cyl).

Below are my troubleshooting 1-7 and results so far :

1 - Spark plugs are old with widened gap (estimate worn to 1.3mm). They were changed. Car drives better but still feel that buckling (especially noticeable at idle).
2 - Coils are relatively new (maybe around 2-3 years). They were changed when I was troubleshooting the infamous "heat soak" issue. However, the coils are not OEM but compatible from Amazon, bought in 10/2018. I tried put back the original OEM set (a mix of Denso\Hitachi), engine running worse. So I conclude the old coils were weak and swapped them back to the Amazon set.
3 - From searching the internet, some says valve adjustment (P1399). I checked and adjusted those valves that are out. Basically, most valves did not need adjustment. Those needed were only off so slightly that I could have assume they are within spec. It seems like the engine smooth out slightly after valve adjustment. However, the intermittent bucking is still there.
4 - Side note: I did the Odyssey fuel rail mod a year ago, it was perfect after the mod and no more heat soak problem.
5 - Yesterday, I started thinking about fuel issue so I put in half a bottle of seafoam and filled up the tank. At first, I felt the slight bucking but after 5-6 miles, engine started running smooth. I was cautiously optimistic but after a little while, the problem happened again so it wasn't a clogged injector.
6 - From the fuel rail mod, I have another spare set of injectors and FPR. I verified the injectors by connecting them to 12v DC and squirt fuel injector cleaner spray through and observed a cone spray pattern. FPR was checked with vacuum.
7 - I thought it wouldn't hurt to swap the injectors and FPR to rule out problems so I did just that this morning. Test drive showed I didn't hit the problem and the car still bucking\slight misses.

I would really like to check fuel pressure but found no way except to spice into fuel line. I want to see if the fuel pressure would drop during bucking. But my thought is the fuel pump is usually very reliable and if it cannot supply enough amount of fuel intermittently during idling, I would see a bad miss when I hit the gas pedal hard (but I can accelerate without problems). My idling LTFT is 0.78% to 1.56% and STFT is 0.78% going up to 3-4% or down to -2.5%. While at 2k rpm, LTFT is 1.56% and STFT goes from about -3% to 5 or 6%. With this numbers, I don't think fuel is the issue.

While I had the intake plenum off, I checked the EGR port and it wasn't block. Few years back, I cleaned it well and it is still good.

I monitored OBD2 data while driving test. I not not knowledgeable enough to be able to point out any problems from the live data. At one time, I monitored the upstream O2 sensor voltage (car not moving, idling). I was expecting a clean oscillating graph but it wasn't. It wasn't anything looking like a sine wave. Sometimes, it goes longer before the voltage change.

So at this point, I am at a lost. There are only a few items left on the table: O2 sensor, MAP sensor, fuel pump, EGR itself. Or maybe coil pack again ??

Also, when I measure the resistance on the Amazon coil pack and compare to the original Denso\Hitachi, they measured differently. Even measuring Denso and Hitachi, they are different.

Any input is highly appreciated !

 

If your accord uses the wide-band a/f sensor instead of an O2 sensor, the voltage reading will be constant.

Since you have a scanner, let us know the short and long term fuel trim when the engine is warmed up to operating temperature at idle. Also let us know the short and long term fuel trim when increasing the rpm to about 2000. It may give you a direction on troubleshooting the issue.

 

Thanks PAhonda for your response. I read enough of your replies so I know you will ask for fuel trims. I have them posted in my original post but I think I was so long-winded so you may have overlooked. I'm reposting the fuel trims here -

Warmed up engine condition:
"My idling LTFT is 0.78% to 1.56% and STFT is 0.78% going up to 3-4% or down to -2.5%. While at 2k rpm, LTFT is 1.56% and STFT goes from about -3% to 5 or 6%."

Fuel trim numbers aren't pointing me towards a fuel system issue. Don't you think ?

I don't think my O2 sensor is wide band. It has 4 wires only.

I was totally expecting comments on me not using OEM coil packs. I was just doing a bit more research on coil pack failure and leaning towards failing coil packs. They were used with the worn out plugs with wide gaps for quite some time (prob over 10k miles). The higher secondary voltage required to just a spark on the worn plugs may have caused failure on the coil pack.

Thanks.

 

Sorry I missed the LTFT and STFT. This doesn't look like a fuel issue.

The wide band a/f sensor also has 4 wires and it looks like your car uses a regular O2 sensor. I'd assume your PCM would set a code if it was getting constant voltage.

What is the specific code you are getting when you only have one cylinder misfire code? When you get a code for a specific cylinder, has it been for the same cylinder before and after the spark plug, coil, and injector swaps?

 

Following were the events in chronicle sequence:-

First code pull before plug, injector etc: P1399, P0300, P0305, P0303
Second code pull before plug, injector but with worn out Denso\Hitachi coils: P1399, P0300, P0305, P0302
Reinstalled Amazon coil pack so car is driveable
Then replaced with new OEM plugs - car runs a lot better, much less occurrence of CEL but slight misses\bucking still noticeable (intermittently happening, either cold or warm engine)
It only threw once CEL since - P1399, P0302 (single cyl only this time)
Replaced injector & FPR with known good set - no change, still slight miss intermittently (either cold or warmed engine, no particular pattern)
Ran out of ideas, measured fuel trims and posted to the forum...

 

So while still thinking what to do next, I checked the TPS voltage just to rule out and it is fine.

Since I'm now leaning towards back to an electrical issue after ruling out fuel issue. The last code was P0302 cyl #2, I thought there is nothing to lose and I should try putting back an old known good coil in #2. From the old batch, I set aside those had set a code. Then I chose the best looking coil (a Denso) from the remaining good ones and swap it into cyl #2 position. Test drove for 10 mins and so far no missing.

Will test drive car for a few more days and report back here.

 

I promised the forum to report back.

After changed out the coil on cyl #2 to an old Denso and driven the car for a few day (combination of long and short trips), I concluded it was due to failing aftermarket china made coils. The chinese coils were used for 2 years and have an almost perfect 5 stars review on Amazon. However, durablility is the issue. I guess most of the good reviews were written not too long after installation.

I'm going to order a set of new coils (debating between Denso and NGK) from rockauto.

Lesson learned.

 

Normally people buy the Chinese coils just to get by until they can afford the OEMs. Honda is notorious for needing the OEM installed. Denso is the ones you should get. Although NGK is a good brand I've just never used them because I know how sensitive Honda system is.

 

I'm also leaning towards Denso. I chatted with an NGK rep asking if their coils are made in China and they confirmed that. But I cannot find anythere in Denso I can ask the country of origin. However, I found a news article about Denso opening a plant for parts production in China therefore I assume Denso coils are not Japan made.

I searched the internet and really cannot find any discussion about Denso vs NGK coils.

 

I don't think you will find anything comparing the two because Denso is the OEM product and NGK is aftermarket. Since the dealership will install Honda/Denso coils there is no comparison.