I've had a persistent issue for some time with my Accord that, for the life of me, I cannot figure out. If anything, the symptoms listed below have become more acute.

Issue(s):
Primary Codes (i.e. come on with every CEL reset): p0301, p0302, p0303, p0304, p0305, p0306, P1399
Secondary Codes (i.e. sometimes come on after CEL resets): P0507, P0421

Symptoms/Characteristics:

• Rough idle at startup (particularly bad when the weather and engine are cold);
  • will go away almost all of the time when the engine is warm.
  • Rough idle is more acute when the car is in gear; not as severe when in Park or Neutral.
• Get the blinking CEL (i.e. misfire), particularly when coasting at speeds at or below 40 MPH - BUT I don't feel the engine shudder of a cylinder misfire.
  • Will also happen when coasting in Neutral (the idle RPM issue, below, also can occur in Neutral when coasting).
• The engine, when accelerating or at cruising speed, feels completely normal, particularly when it's warmed up.
• At idle, RPM range fluctuates wildly (250-1000 RPM); engine shudders at the ~250RPM low range)
  • Particularly bad when the A/C compressor is running; at times, it will maintain a steady RPM but below the standard ~750 RPM
  • It can be at the normal (750 range), especially when the engine is warmed up
  • The fluctuating RPM doesn't pop a CEL.
  • There's been a few times, very recently, where the engine stalled in these conditions.
• Poor MPG (normally will get 360+ miles p/tank - currently getting 280-300 miles p/tank)
  • The engine has been underpowered when going up inclines in CO (i.e. pedal to the floor)

Work done to date with no improvement in symptoms:

• New Spark plugs (all 6)
• New ignition coils (all 6)
• Replace the Idle Air Control valve (FWIW, not replaced with a Denso OEM part due to supply chain issues)
• Replaced spark tube & valve cover gaskets (they were leaking oil)
• Tested alternator (a bit on the low side, 14.0-14.1 Volts)
• Timing belt only has 40K on it.

Thoughts, Ideas? Anyone with prior experience with this in their v6, J-series Honda engine?

 

Do you have a scanner that can read live data?

 

I only have a low tech blue tooth scanner. Not good enough to get live data, unfortunately.

I have another update from this evening(after someone else recommended a test to do first):Conducted a compression test (just a dry test this evening). All 6 cylinders came out between 145-157 PSI. So far, so good.

With pulling the spark plugs out, I looked at them. Nothing looked abnormal on the tips, which is what I would expect after replacing the leaky spark tube seals and valve cover gasket 18 months ago (photo included via link - https://photos.app.goo.gl/WgRK6QsHi7nWmdd78).

 

What brand blue tooth scanner do you have? Does it come with software or do you use an app like torque or torque pro?

 

Panlong OBD2 Scanner Bluetooth OBDII Diagnostic Tool Car Code Reader

 

Your obd2 blue tooth can work with the torque lite/pro app and some others. I believe the free torque lite app can read live data. If not, the torque pro version can read live data and costs ~$7.

Look for stft and ltft. You have a V6, so you should have a bank 1 and bank 2 (b1 and b2) for short term and long term, This can tell you if your are running rich or lean and give you a direction for testing on your car.

 

I just tried a different OBD-II app as it was free, but it worked for live data.
Oddly, it only had ST and LT fuel trims for Bank 1. (???) Anyways, ST varies significantly when at idle (especially with the A/C running) or coasting. I took some screenshots of the graph. Generally, it was negative by a good margin - sometimes up to -18%. LT Fuel Trim is at ~7.03%. I took some screenshots.

 

Bumping this forum up to see what I should look at next since we know compression is good.
FYI, did a quick test on the IACV by unplugging it. RPMs surged, as they should have.

 

Look at the mass air flow sensor readings in g/s and the intake air temp sensor values. Let us know what values they are showing.

 

I'll see if i can get this tonight.

On a different note, i "splurged" on the Torque Pro app with hope of better live data options. Interestingly, the Torque Pro app doesn't show any option to do Bank 2 data - the OBD2 is showing that there is no bank 2 data.

FWIW, i *think* the issue is in Bank 2. All ignition coils show spark, all fuel injectors show signs of working, but when unplugging the coils individually the engine doesn't react in the same way as Bank 1 ignition coils.