Help With Your Honda Hot Start Problem

The most common time to have a no-start condition in a Honda is after a hot sit, like when you pull in for gas or a quick run into a store. If you are unsure you can duplicate this condition at home: Use a piece of stiff wire to hold the throttle linkage at a set position and set the engine speed at 2,500 rpm. Let the engine run for about 20 minutes with the hood closed. Then remove the wire and turn the engine off. Let it sit for five to 10 minutes, then try to restart the engine several times. If the engine doesn't start, turn the key on. The check engine light will come on for two seconds and go out. You should hear the fuel pump run during the two seconds. When the light goes out you should hear the main relay click. If it doesn't click, check terminal seven on the main relay (fuel pump) for power and terminal eight (computer) for ground. If you have no power and you have ground, the main relay is bad.

Although the problem is the same, different models have different specifics if the main relay is bad. On an Accord, you will lose fuel pressure. If it's bad on a Civic, you will lose power to the injectors and the fuel pump but you may not lose fuel pressure because the injectors can't open without power. When the main relay goes bad and there isn't any voltage at the injectors, it will set a code 16 for an injector because the computer doesn't read voltage on the ground side of the injector.
Before you dive in too fast, it's also possible that the car has more than one thing causing a hard start. You could also have a bad ignition switch, a bad igniter or a bad ignition coil. To test for spark, you should first perform a simple spark test, then you can test the coil itself. Unfortunately to test the igniter itself, you need an automotive oscilloscope, something you won't really need too much in a home shop.

The main relay will give you the same symptoms as a bad coil or a bad igniter. A main relay usually only fails when the weather is really hot. You might have a hard start every now and then, but not enough to really cause you any concern, but when an igniter or a coil fails, the car won't start at all until it cools down.

If you've determined that the culprit could be the main relay, you should do a Honda Main Relay Test to be sure. There's nothing worse than replacing an expensive electrical part only to find that it wasn't the problem in the first place. Don't forget, many parts suppliers have a "no returns" policy on anything electronic.