I have a 1990 Honda Civic. I have been having this odd intermittent starting problem for a year now. It ONLY has difficulty starting when the weather is hot. In the morning, everning/night, and during the fall/winter it starts like a champ, on the first crank every time. But during the hottest parts of the day it just won't. It sounds like it wants to but just doesn't. Now if I wait 30 minutes or so and try again it starts normally.

I have gotten a tune up, changed the fuel filter, air filter, flushed the radiator, and checked the battery but the problem persists in this very predictible way. I read from someone else's post that for many Hondas the Main Relay is the culprit so last weekend I bought a new main relay from the auto parts store. Much to my dismay, the starting problem persists.

What am I not thinking of?? What else could pose such a specific problem--starting problems during the hottest part of the day only, reliably starts 30 minutes after first attempt, never a problem at night or during winter?