I recently changed the timing belt on my 1998 Honda Civic EX 145,000 mi. The top plastic timing belt cover was difficult to remove and when it was finally pried off, I noted that there was a great deal of melted plastic inside. The old did not show any signs of heat damage but it may have been changed previously. Any idea of how the plastic could have melted in this relatively low heat area of the engine?

Are you the only owner of this vehicle(not bought used)
Sometimes if the engine is overheated it will melt the plastic inside the timing belt cover, I've seen that a couple of times. If everytihing is working now don't worry about it just make sure that there is no plastic rubbing on the timming belt.

The car did overheat on my son twice. I noticed low coolant after 2 months of operation afterwards. I replaced the radiator which finally showed a pinhole leak.
Thank you for the response.