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Main Relay or Vapor Lock?

1990 honda civic lx

Won’t start after being in heat/hot sun. Starts after 5 min or when it cools down.

Main relay. Easy to swap. Very common on that era Civic.

Buy a main relay and replace it. $30 - $40. You can replace it yourself.

If the main relay is not the problem then it could be a faulty ignition switch. Some Hondas are under a Recall for switch problems (yours is not) but by the same token, many non-recalled vehicles of the same make also suffer the same problems.

I’m betting on the main fuel injection relay. Under the dashboard. Common problem with older Hondas.