Vapor lock is not the issue, but you could have a fuel problem. You'll need to provide a more thorough description of how it acts though.
If it slowly looses power on the highway, especially if it occurs during coasting, you could have a plugged pickup screen in the fuel tank. This screen must pass the most fuel when you are coasting, and the least amount of fuel when accelerating and cruising. Yes, I know that sounds backwards, but it's true.
Fuel delivery problems almost always cause the engine to die slowly over a period of a couple of seconds or longer. Electrical problems generally cause the engine to die instantly; the same as if you turned off the ignition switch. When this happens, listen for the sound of the fuel pump running for two seconds when you turn the ignition switch on. If you can't hear it, have someone listen by the tank or gas cap. If the pump does not run for two seconds and then turn off, suspect a problem with the engine computer or Automatic Shutdown (ASD) relay.
If you hear that two-second burp from the pump, check for spark at one of the plug wires while someone cranks the engine. Chances are it will be missing. Without the luxury of observing other symptoms or being able to perform other tests, an educated guess would be a defective camshaft position sensor or crankshaft position sensor. The cam sensor has three wires and sits right in front on top of the 3.5L engine. This type of sensor often fails when it gets warm.
The engine computer turns the ASD relay on for two seconds when you turn the ignition switch to Run. That runs the fuel pump to insure fuel pressure is up and ready to start. After that, the computer turns the relay on again when it sees engine rotation, (cranking or running). It knows the engine is rotating by the pulses from the cam and crank sensors. When the ASD relay turns on, it sends voltage to the ignition coils, fuel injectors, alternator, and fuel pump or pump relay. When the sensor stops working, the engine computer thinks the engine stopped so it turns those things off. I never had to replace a crankshaft sensor, but the camshaft sensors do fail now and then.
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Monday, March 23rd, 2009 AT 3:26 AM