My 1993 Civic has new-onset failure to start. The problem has occurred
sporadically and only on very hot days. A friend swears it's vapor
lock; previous posts to this group indicate that vapor lock is very
unlikely and suggests a main relay problem.

* *{Additonal data:
Several times the problem was solved by waiting til the next morning
and trying again.
Two other times we've put STP Gas Treatment in the tank and the car
started up immediately.
The problem never occurred until this year. The first time the car
wouldn't start, the positive battery terminal was severely corroded.
Cleaning it did not start the car. The car started right up when the
mechanic checked it hours later. He noted that, unrelated, the head
gasket needed to be replaced, and he did that.}


My questions: if it were a main relay, why would the problem only occur
on scorching hot days? In any case, why would such a problem have
sudden onset? (I'm the original owner and this has never happened
before this year.) Could it have anything to do with ethanol
supplemented gasoline? Thanks