From what I can tell, the 2004 doesn't have an EGR valve, but my symptoms seem to be the same--engine shutters at idle, but will smooth out under full accelerator. I was sure it was an EGR problem after watching several videos on the topic for a Gen 3 model--couldn't figure out why I couldn't find info on the Gen 2 models, until I removed the air intake assy and tried to find the silly EGR without joy. So now I'm stumped...
Engines could have rough-running problems for decades before there even were EGR systems. The possible reasons are numerous, and somebody just needs to work through them for the specific engine and find out what is going on with it. What has happened over in the Gen 3 forums, where the engines do have EGR, is there's a contingent that leaps to "EGR!" whenever a Gen 3 engine runs rough, with the result now that even people with EGR-less engines end up describing any rough running as "like EGR problem" and feeling stumped 'cause there's no EGR, because of seeing too many Gen 3 forum posts or videos.
Thanks everyone. Yes I will try to follow the numerous recommendations particularly with the spark plugs and ignition coils and the fault codes. It's been doing this for awhile, and it is our seasonal car that gets left in NY for many months at a time. The inverter is slow to charge the battery sometimes, but I eventually get back up to 6 bars on the battery--haven't seen a full 8 in awhile, and it regularly goes down to 1 or 2 bars. I did record some misfire codes last winter, but I can't recall which cylinder or if it was erratically across the cylinders. I will check that again and swap the ignition wires and see what happens. Unfortunately, I did not refresh my posting and did not see all your responses until it was too late--now it will be another 6 weeks or so before I can do any troubleshooting again.
An ageing Prius is probably not a good choice of vehicle that will be left for many months without driving. To be clear, you do not swap the wires. You need to swap the actual coils between cylinders.
