Two problems which may or may not be related.

Problem #1

My 2004 TL developed a misfire code (P0300).

My engine has difficulty accelerating with heavy loads, whether it’s driving up a hill or pressing accelerator pedal more than a little.

It will stutter and flash the check engine light for a few seconds after the load lightens. When I check under the hood, I notice a lot of smoke emerging from my front catalytic converter.

I’ve individually unplugged the coils to see if there was a change in performance, but the engine started struggling with every coil I unplugged, so I guess they’re all still working.



Problem #2

A high-pitched noise is coming from somewhere beneath the upper intake, and I don’t know what’s causing it.

The noise lingers for a couple of seconds after I switch off the ignition.

Could a bad fuel injector do all this?

See below:

 

It sounds like you might have two problems now.

The sluggish acceleration might be caused by a clogged cat, which might have been caused by the misfire. The flashing check engine light means "Catalyst damaging misfire". Most likely due to fuel being burned in the exhaust manifold. You will need to check it after fixing the misfire if it's still sluggish.

The reason you probably did not notice a change in performance when unplugging coils is because of the IAC. The IAC compensates for the change.


Yes, it could be caused by a bad injector. I would start by diagnosing the misfire and focus on checking Bank 2 (front) injectors.

The simplest way to test them is by measuring the resistance. A good injector should be 10-13 ohms, and draw about 1 amp.

That method requires removing the upper plenum.

Other methods require a bi-directional scanner and pressure gauge, or graphing meter/scope with a current probe.

 

The problems are fixed.

The noise was indeed a bad injector. I got a fuel rail from a junked 2007 Honda Pilot and used a few of the injectors from that to replace mine. I didn't want to have to remove the upper intake again in case I picked the wrong injector to replace so I just replaced all 3 of them in the front.

My misfire problem was harder to diagnose, and I almost gave up on the car.
It was caused by a bad coil, after all. The engine ran fine under idle and light load but would only exhibit trouble when going up a hill or pressing the accelerator more than 1/3.
I purchased a coil from a junked 2007 Accord. One by one, I installed it into a spark plug well and took it for drive. There had been no improvement until I came to the one coil in the engine I hadn't replaced, but at that point I doubted it would have done any good. It ended up being cylinder #6.

I was glad to figure it out, but thoroughly ticked-off that it had to be the last one.

 

Glad it worked out.

Thanks for providing an update.