2001 Honda Accord LX, 4-cylinder (F23A1), 145,000 miles.

Fault codes are P0300, P0301, P0302, P0303, P0304, and P1399. Components replaced in the past 18 months (for unrelated reasons): distributor cap, plugs, plug wires, catalytic converter, PCV valve, idle air control valve.

At idle, the misfire is nasty -- bad enough that it can heard, felt, and seen (in observing the car vibrating). Acceleration from idle is poor, but once above ~2000rpm, the misfire appears to clear up and the engine runs just the way you would expect it to.

The misfire begins immediately upon starting the engine and the problem is just as bad with the engine at operating temperature as it is when starting cold. Load conditions also do not have an effect on the misfire: in gear or in neutral, compressor on or off, etc, it stays the same.

Cylinder compression values are good and even across all cylinders. Fuel pressure test came up good. Searched for vacuum leaks (using propane) and found nothing. Can't find any evidence of arcing anywhere from the distributor cap down to the plugs or inside the distributor, itself.

I'm not a mechanic. I have some ideas, but I don't want to start blindly slinging parts. Thoughts? Is it possible that the EGR valve is stuck open? Crank position sensor? Should I set the car on fire?

Any advice appreciated. Thanks in advance.

I would suggest verifying that the valves are adjusted properly as well. If they have not been done at 90K miles they are more than due. I prefer to check them every 30Kmiles.

Possibly the EGR valve is cracked open just a tad, pull it off and make sure no carbon is keeping the valve open. Exercise the valve open and closed a few times to make sure it opens/closes smoothly.
Some folks have luck soaking the bottom half of the valve itself in WD-40, but you don't want any cleaner to get up into the electrical solenoid as this may remove the varnish on the windings and cause a dead short.
Disconnecting the electrical connector should put the engine into a limp mode. If there is a mechanical problem, and not sensor, the engine will continue to miss.
If it still runs rough then the mechanical side of the EGR valve is probably not sealing correctly anymore. Either from too much carbon/soot blocking the seat or a weak/broken return spring.
The later powered solenoid EGR valves are a bit more complicated than earlier vacuum diaphragm units.
https://www.insightcentral.net/forum...air-ideas.html