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Discussion Starter #1 (Edited)
(This is after doing your due diligence of checking for vacuum leaks and keeping up with tune-ups)

You might want to adjust your valves sooner than later since it is a common problem on the Honda 3.5 liter engines...

What's happening is that your exhaust valve clearance gets tight due to wear on the metal contact. This causes the valve to open sooner and close later than it should. Closing later allows a small amount of unburned fuel to go into the exhaust. It won't always trip the P0300 Check Engine light because it's such a small amount of unburned fuel passing through.

The tight exhaust valves will cause your short term fuel trim will be around 14-20% where it should be close to 0%. The check engine light turns on when it reaches passed 20%. So you'll usually see the light come on when the engine is cold or is at idle for a good amount of time. I recommend you get a good scan tool to check the live data and see where your short term fuel trim is reading at when the engine is at operating temperature. You won't know you have unburned fuel passing into your exhaust without checking the STFT live data. You can go a long time without the CEL coming on because it's such a small of fuel passing through.

This unburned fuel causes carbon build up to go through your EGR and back into your intake. The carbon builds up in your intake valves to cause a looser clearance. The unburned fuel will also cause carbon build up in your EGR ports, throttle body, intake manifold, oxygen sensors, and catalytic converter. Depending on how long you go without adjusting your valves, you might build up enough carbon in your catalytic coverter and get the P0420 code.

There's also an additional factor that is causing a high short term fuel trim at IDLE. I found this out after making the valve adjustment and found that my STFT was still high, but only at idle. The fix was replacing the leaking Hydraulic Timing Belt Tensioner. This I can't really explain why it would cause a high STFT but it did. It made a drastic change on the engine perfermance once replaced.

If you want to fully fix the problem, then adjust the valves, clean your intake and throttle body, clean the EGR ports, replace the leaking Timing Belt Tensioner, replace/clean your oxygen sensors, and do the idle relearn procedure. You might also want to run Seafoam before adjusting the valves to get rid of any loose carbon off the intake valves. You can also run lacquer thinner to clean your catalytic converters (this is just a myth on how to clean them though).

I HIGHLY recommend getting a good scan tool to read your live data and see where your STFT is at. I can't stress enough how important it's going to be for you to keep tabs on the STFT. I use BlueDriver which connects to your phone through bluetooth and updates when new information is available. You won't know if it's fully fixed until you get your STFT below 8%!

If you have already adjusted your valves, I still recommend checking your STFT. I wouldn't have found out about the Timing Belt Tensioner if I haven't checked my live data.

Please let me know if this has helped you and feel free to chime in with any other pointers. Attached is the TSB and instructions on how to adjust the valves.
 

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