I had a problem with the fuel pump coming on a couple of times on hot days. i replaced it this past weekend and the issue came back. one day when i was having the problem, my uncle suggested i vent the gas tank (loosen the gas cap). i did and the car started right up. i assumed it was vapor lock, so i kept my gas cap secure but not snug. i'm having the same problem again today, only the weather is cool and rainy and the gas tank is vented. i can't hear the fuel pump kick on.
Fuel pump operation on your impala is controlled by the computer. It activates the fuel pump relay when ignition is turned on, but only for a few seconds. This is called the fuel pump prime. When the computer senses a crank signal (upon start up) it will activate the fuel pump relay constant. I have found more problems with the fuel pump relay than anything. Vapor lock will not cause a no start unless it is in the fuel lines. GM systems use the manifold air temp sensor to allow for quick starts when hot......increases fuel injector on time. An intermittant fuel pump relay or bad pump will cause a no start any weather.
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