I have a 99 Honda Accord that only starts sometimes. My problem seems to be when it is hot outside. I've replaced: fuel pump, ignition spark control module, cap & rotor, and coil. I've tested for voltage at the fuel pump plug in the trunk. When the key is turned "on," I get voltage for about 2 seconds, then it goes away. If I keep turning the ignition switch on and off many times, sometimes the voltage will stays. This is a very frustrating problem, can anybody offer a solution?

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Sounds like a classic Honda main relay problem, when it gets hot inside the car. Then, printed circuit board cold solder joints expand and fail to conduct electricity. Simply replace the relay:

Zip Code Entry at PartsAmerica.com (http://www.partsamerica.com/ProductList.aspx?PartType=598&PTSet=A&SearchFor=Fuel+Injection+Relay)

It's located left of the steering column.

Turning ignition key on and off will help you locate the main relay, by allowing you to hear and feel it "click." Main relays often have 7 wires going to them.

If the problem persists, reading sections A and B below should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563.html