Here's an article I found that shows one auto expert's view on vapor lock and how to solve it. Here's the URL:
Vapor Lock Problem
My problem is with a '92 Ford Explorer. When I drive it long enough to get the engine hot, it won't start if I shut it off and it sits for a few minutes. My temperature gauge doesn't give a high reading, but it does seem hot when I pop the hood. When it doesn't start, the fuel pump doesn't seem to engage when I turn the key without turning it enough to engage the starter. The engine turns over fine, but it just doesn't fire. To me it seems like it isn't getting any fuel. I have tried holding the pedal down and pumping the accelerator, even though it is fuel-injected, to try and get some gas to the cylinders. Another possible symptom is that the brake pedal seems to get pressurized and I can't press it down, but is this just caused by turning the engine over many times without it firing? The truck runs great otherwise. And it will start if I give it ample time to cool down. I had the fuel pump replaced last year and I changed the fuel pressure regulator myself last year, so I don't think either of those are the problem. My guess is it is flooding or something like a sensor or a chip is keeping the fuel pump from engaging because the engine is hot.
ACould this be a classic case of vapor lock? Very possibly. Vapor lock used to be a common affliction with carbureted engines during summertime heat. Engine and exhaust heat would raise temperatures under the hood and near the fuel line until the fuel literally boiled. As the fuel boiled in the fuel line, the fuel pump could no longer pump it. If the engine was running at the time -- not at all uncommon -- the symptom was a complete loss of power. In many cases the engine continued to idle, but would not react to opening the throttle at all. If the engine vapor locked after shutting it down hot, the symptoms were precisely as you've described -- it will crank over but not fire. Can vapor lock occur with modern fuel-injected engines? Absolutely.
Reflecting on the basics -- an engine needs air, fuel and ignition to run -- your best course of action is to check for fuel and spark while cranking the engine in the "no-start" condition. An old spark plug or timing light can be used to check for spark. With the key off, adding a small amount of fuel or volatile aerosol through the throttle body or a vacuum line, then cranking the engine, should tell if it is a fuel starvation problem.
If the engine starts and runs for a couple of seconds on the fuel you added, it's either vapor lock or some continuing issue with the fuel pump, fuel pump relay or computer system.
If you determine it is a loss of fuel pressure, try carrying a large squirt bottle of water in the car so that when this scenario occurs you can spray individual fuel system components under the hood such as the fuel rail, pressure regulator, throttle body, etc., to rapidly cool them off, then try restarting the engine. If you find you can get the engine running again by cooling, for example, the fuel rail, then you've identified vapor lock as the problem.
The only solution is to lower underhood operating temperatures by servicing the cooling system and cleaning debris from in front of the radiator and A/C condenser, and trying a different source of fuel. You might find a fuel with a slightly lower vapor pressure that will reduce the potential for vapor lock.
Also, here's a tip. When you know you're only going to stop for a few moments, pop the hood open and let the engine idle for a minute or so before you shut it off. This will help carry some of the engine heat into the radiator and dissipate it into the atmosphere. Leave the hood open on the safety catch until you return to the car, to continue the cooling process. Remember to shut the hood before you drive away.
As for the "hardness" in the brake pedal, not to worry, this occurs because there's not enough engine vacuum available for power assist to the brakes while you're cranking the engine.
And finally, it is possible that the fuel pump relay is not being energized or that the pump's not running when you first turn the key on, but remember, the pump will only run for about two seconds with the key on, unless you begin cranking the engine.
