Hyundai: Accent 1.5 engine pre-ignition ("pinging")

Ok guys, I've had this problem for the last several months under
medium to hard acceleration. I replaced the plugs, the air filter and
fuel filter and have bought gasoline in several cities, but the
problem remains. I wouldn't call it a severe problem but it is
annoying. Any ideas on what to try next?

Thanks,
Okie

It's possible the knock sensor may not be working. It's also possible
there may be some carbon buildup in the cylinders and on the valves. I'd
go to my local GM dealer and purchase some aerosol top engine cleaner and
use it per the instructions on the can. You may need to unplug the air
flow sensor to keep the car running. This will set a check engine lamp
that will need to be cleared. If that doesn't work, then I might consider
taking it to be diagnosed or just using premium fuel.