2008 Honda Odyssey EX; Lean Condition

I've been struggling with this car over the last 6-8 months or so. Here are the details and history:

2008 Honda Odyssey EX 3.5L no VCM (cylinder pause system)
172K miles

History: Last year at 132K (yes she put 40K on in a year) the owner complained of "chugging." I drove it and felt a slight hesitation but not very noticeable. In looking at live data I found fuel trims up in the 10-15% range on both short and long terms that don't change significantly with RPM. I've seen this before on these Honda V6's, and with the mileage I suggested to change all four O2 sensors. On the last three of these that I worked on, including my own similar vehicle, when exhausting all the normal lean condition checks, replacing the O2's brought the fuel trims back to normal. So, we replaced the O2's with NTK (the OE sensors.) Upon my scan of the vehicle after replacing the sensors the fuel trims were still elevated. Resetting the PCM afterward didn't help, either. However, I gave the vehicle back and the customer was thrilled because the "chugging" was gone.

I let the vehicle go and fast forward to today. She came back for an oil change but she complained of an intermittent check engine light and "slow starting." Now I haven't been able to duplicate this "slow starting" and when I question her on exactly what that means I can't get a good answer. I think she's complaining of a long crank time but I noticed that, with a good battery, the starter did seem to crank significantly slower than my own 08 Odyssey. At 167K miles on the original starter I wasn't too surprised. I also scanned the vehicle and found that the code it has been throwing intermittently is a P0420. In looking at the data I found that the fuel trims are still elevated in that 10-15% range on both short and long term. Not wanting to throw parts I smoked tested the intake and found no leaks. I also checked fuel pressure and found good pressure at about 60psi that was steady and solid. When I compared the live data to a known good (my own vehicle) it appeared the MAF was under-reporting a bit and with the mileage we opted to change it. We also opted to change the starter as it was apparent it was starting to slow down and voltage drop tests showed the cables were fine. I also recommended we do a valve adjustment at that mileage as she said it has never been done. I also cleaned the throttle body and relearned the idle. During all this work I found the intake tube cracked and replaced that, as well, and thought that might have been the issue all along. The new starter spun the engine over much faster so that was a good call but not sure it was the issue she has been complaining about.

After all this, I reset the PCM and the fuel trims jumped right back up to the 10-15% range on both short and long term. I feel like I've fired the parts cannon at this except that everything I've replaced has actually been a problem but not the problem. Again, it's not throwing any lean codes but does have an intermittent P0420 code. So, before we eventually change the cat I'd like to get to the bottom of why it's running moderately lean as the last thing I want to do is spend the money to replace the cat only to have the issue return because I didn't do my due diligence and find the cause of the lean condition.

What am I missing that will cause this J35 engine to run lean at all RPM's? Is this intermittent "slow start" that she is complaining about that I've been unable to duplicate, a clue? I asked her when this slow start occurs (cold or warm, 1st drive of the day, etc.) and she hasn't been able to tell me. All she says is it's just "sometimes." Other than this random slow start and P0420 code she has no other complaints. I'm thinking maybe dirty fuel injectors but without some significant disassembly I don't know how I can actually check that. I don't want to fire injectors from the parts cannon.

What am I missing?

Thanks,
John

Have you performed a compression test?

Long start time and ecu adding fuel (LTFT & STFT), could be a mechanical issue.

Any TSB's for this? Software versions good? IS there a recommended fuel injection cleaning mileage that maybe hasn't been done yet?

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