I've got a 2008 Accord, Japanese model, with a 2.4l i-VTEC (K24Z2), 58,000 miles. Totally stock. I guess this is more like an Acura TSX than an Accord, see here
After about an hour of highway driving, often heavily loaded with my entire family of 4 and luggage, the engine begins to rev limit. I begin to notice it at about 4500 RPM, but it steadily declines, over a period of about 20 minutes, until it as low as 2000 RPM.
I'm in a warm place, and temperatures are often 85 F or higher. Cool mornings do seem to delay the problem, but only delay by about an hour, not remove.
By rev limit, I mean that the fuel is cut to the engine at that RPM. The tach needle bounces around the limit value until you let your foot of the gas and go to a lower RPM. Look into LIMP mode if you're unfamiliar with this behavior.
Restarting the engine restarts the process. I get about 15-20 minutes, depending on temperatures and how long I've been driving, to drive with higher rev limits, but eventually it gets down to around 2000 and I have to restart again. Several times, I've had to limp home by putting the car into neutral and restarting the engine while it coasts, every 15 minutes, for 1-2 hours. I do this especially on approaches to hills or when needing to overtake a vehicle.
Around town, it's cool. No problem for 15-30 minute drives.
Now, the hard part. There are no error codes! The check engine light does not engage. I have confirmed that the CEL does light by starting it with the VTEC solenoid unplugged. The oil level is perfectly between high and low. The temperature gauge looks great, well under the half way point. Fluids are good. I recently changed the oil to OEM synthetic 5W-20 and filter. I recently replaced the VTEC solenoid. I've checked ground wires and looked at all major sensor connections, especially the oil pressure sensors. Nothing amiss. The engine compartment is super clean. I've disconnected the negative battery terminal to reset the ECU. In spite of these efforts, the problem remains.
I've located one post of a similar problem on a Honda:
CRV loses power and stutters at high RPM in hot weather.
And seen many other LIMP related problems, but usually only on cars that throw a lot of error codes and are older/heavily modded.
Questions to the community: Any ideas? Alternator, choked catalytic converter, damaged ECU, and timing chain tooth missing are what I think are current possibilities. If you could add to or remove an item on this list based on my description I'd be eternally grateful. Also interested in diagnostic advice. I've got a very limited set of tools here.