Honda rev-limited at 4k RPM, automatic transmission won't shift into 5th gear, no CEL

Question

I have a 2010 Honda Accord EX-L, with the 3.5L V-6 engine. 230k miles, all highway, so I'm still breaking her in.

At the beginning of the Corona shutdown, I replaced the timing belt and water pump. I don't think this is related, as the car behaved fine for ~500 miles after.

I first noticed that something was off last night driving on the surface streets, not exceeding 55 mph. I didn't think much of it, I haven't been driving much since the shutdown... Until trying to overtake another vehicle on the expressway. My car will not exceed 60 mph unless going downhill. It seems the engine is limited to 4,000 RPM and does not feel like it is shifting into 5th gear. There is a slight hesitation shifting into 4th as well, but only if I'm really rough with it. If I had to guess, the ECU is limiting the RPM, not the speed.

Prior to 55 mph, the car behaves normally. Plenty of torque, so I think it's getting enough fuel pressure and the AFR is good.

At idle the engine runs smoothly, so I don't think the timing belt has slipped. I've not taken the cover off to confirm. But I can if needed.

Strangely, there are no trouble codes (except for a faulty O2 sensor, which I've had for 2 or 3 years). I can clear that code and the problem persists. The "D" on the gear selection indicator does not flash.

I drained the trans fluid and replaced it with the proper ATF-Z1. No noticeable change in behavior. Old fluid was dark in color, but no shimmer or metal particulate in the fluid.

I swapped out all of the spark plugs, I had them on hand after noticing they were looking a little rough. No change, though I didn't expect any.

At this point, I'm not entirely sure how to narrow the problem down. If the car was getting put into limp mode, I would expect a trouble code?

The only other thing worth mentioning is that the power steering pump is toast. I need to get one of the hoses (dealer item) which has a leak, replaced. Maybe this is putting an extra parasitic loss on the engine's output torque? But again, I would expect a trouble code...

So what's your gut feel? What steps can I take to troubleshoot?

Answer 1

For those who come after me...

The issue ended up being a blocked catalytic converter. Holding it up to the light it looked fine, you could see easily through the honeycomb.

What ends up happening is that the engine struggles to push air out of the cylinders, thus causing the manifold vacuum to be less than it should be. This tricks the powertrain control module to believe the engine is under load. When the computer believes the engine is under excessive load, it will delay shifting. If you have an OBD scanner, the CALC_LOAD% PID and MAP PID would help identify this trouble. I was seeing CALC_LOAD% values around 80% when the rev-limiting would kick in.

This is why if I took my foot off the gas it would immediately upshift. Perhaps if you're trying to diagnose this on your vehicle, try letting up on the gas and seeing if it will upshift for you. From there, if you pull an O2 sensor, it will provide another exit path for the exhaust. (This would set a trouble code that you would need a scanner to clear.)

In my case, it was the third catalytic converter, not the pre-cats.

Here is the forum entry that finally led me to the solution. https://www.driveaccord.net/threads/low-power-and-bad-at-shifts-due-to-clogged-catalytic-converter.485193/