Does Your Honda Have Trouble Starting When the Engine is Hot?

Honda Hot-Start Hesitation May Be Caused by a Main Relay Problem

2007 Honda S2000
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Honda automobiles are notorious for having troubles with restarting after a fully hot engine has been sitting for five or ten minutes—such as when you've just pulled into a gas station fill-up or when you've run into a grocery store to pick up a few items. 

Testing the Main Relay

A very common reason for this symptom is a problem with the main relay—an electronic device that opens and closes the fuel supply to the engine.

To determine if you indeed have this problem, try the following test: 

  1. Use a piece of stiff wire to hold the throttle linkage at a set position and set the engine speed at about 2,500 rpm.
  2. Let the engine run for about 20 minutes with the hood closed.
  3. Remove the wire from the throttle linkage and turn the engine off. 
  4. Let the engine sit for five to ten minutes, then try to restart the engine several times.
  5.  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.
  6. If you don't hear this clicking sound from the main relay, check terminal seven on the main relay (fuel pump) for power and terminal eight (computer) for ground. If you have no power even though you have a proper ground connection on terminal eight, it means that the main relay is bad. 

    The Effects of a Bad Relay

    Although the problem is the same, different Honda models have different symptoms if the main relay is bad. On an Accord, you will lose fuel pressure. If the main relay bad on a Civic, you will lose power to the injectors and the fuel pump, but you may not lose fuel pressure since the fuel 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 computer message for an injector, because the computer doesn't read voltage on the ground side of the injector.

    Other Possible Causes of Hot Start Problems

    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 that is used so infrequently that you probably don't have one in your home shop.

    A malfunctioning main relay will give you the same symptoms as a bad coil or a bad igniter. But the main relay most often fails when the weather is really hot, while the other possible causes will exhibit the symptom nearly all the time. Although you might have a hard start now and then with a faulty main relay, it is usually not enough to cause you much concern—you can usually get the engine started despite the momentary difficulty. But when an igniter or a coil fails, the car won't start at all until it cools down.

    Before Replacing the Main Relay

    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. A main relay can cost $50 or more, so be sure before you replace it. But If you are fairly sure the main relay is the cause of your hot-start problem, doing the replacement work yourself can save you at least $100 on the cost of service garage labor charges.