I've got a 2002 Dodge Intrepid that is pinging way too much.  I've switched
to premium fuel and that helped a bit at first, but it is still pinging too
much.  Vehicle has 145K miles.

A mechanic who saw the car suggested that the combustion chamber has taken
on carbon and that has increased the compression ratio.

There really isn't much to maintain on that engine.  The plugs are new; the
engine doesn't have a distributor or wires; air filter is new.  I don't
think there are any air/vacuum leaks as it idles smoothly.  Replacing the
plugs didn't make any difference; try ones in a colder heat range? Does the
engine have a knock sensor?  Web searches seem to indicate that Dodge quit
using one in 2002.