I just changed the sparkplugs on my 2001 accord with NGK plugs. After the changed the car idles around 500 rpm where before it idled around 800. I replaced the plugs without a torque wrench and wanted to leave them looser rather than strip the threads. Could this be the cause?

 

Well the plugs are not much, around 13 ft lbs.....not much.

What I would do is a "idle re-learn". There is a post in the "Common DIY" threads and info sticky at the top of this section....Jim posted it.

Read it but "my simple version" - Jim don't flame me - is this. Pull the backup fuse to the ECU/PCM....hope you have the code for the radio if factory.....start the car and do not touch the gas pedal......let it run until the cooling fans cycle at least once.

If that/this does not help let us know and we can get/go into more.