My truck will not keep running it shuts down as soon as u turn the key

dont know why changed fuel pump if total power failure but need to test electrical for short [ will need a meter ] start with battery test and check wiring and connections and wotk your way thru ignition system and starter system looking for reason system has a direct short

Does it keep cranking like when first starting or is it running normally and when you turn off the key it just keeps running if it's the first one the solenoid on the fender is N/G the engines running but the starter is still cranking trying to turn the engine over but it's already running///// next if it's running normally after starting no grinding from the starter being engaged like I just described then the ignition switch on the steering column is N/G//// THIS IS NOT THE IGNITION KEY SWITCH THAT YOU PUT THE KEY IN TO START THE TRUCK THIS IS LOCATED DOWN ABOUT HALFWAY BETWEEN THE STEERING WHEEL AND YOUR FEET THERE IS A ROD THAT COMES FROM THE KEY SWITCH WITH THE KEY THAT WHEN YOU TURN THE KEY YOU ARE MOVING THIS ROD WHICH MAKES THE SWITCH MOVE TO THE POSITION TO START THE TRUCK WHEN YOU LET GO THE SPRING INSIDE LET'S THE SWITCH MOVE TO THE RUN POSITION SO WHAT'S WRONG HERE IS THE SWITCH IS NOT OPENING WHEN YOU TURN THE KEY TO THE OFF POSITION IF IT'S A NEW SWITCH YOU HAVE TO ADJUST IT TO GET IT TO SHUT OFF THE RIGHT WAY IF IT'S AN OLD ONE REPLACE IT BUT THIS COULD HAVE BEEN REPLACED BEFORE AND THE FASTENERS LOSSENED AND MIGHT NEED TO BE ADJUSTED AGAIN AND TIGHTENED TO HOLD IT IN PLACE GOOD LUCK! STEPHEN

Could be several things including one of the oxygen sensors, one of the injectors, the coils, or the engine computer.
You will probably need to get a shop manual to get all of the wiring diagrams connected with that circuit. And I would suggest you use a circuit breaker to check each one instead of fuses.
You'll have to disconnect each component until you find the short.

Use fuel pressure gauge to see if pressure bleeds off as soon as you shut engine down. Fuel pump check valve is supposed to keep pressure in system.

TAKE TO DEALERSHIP LET THEM CODE SCAN VECHICLE . YOU NEED DIAGNOSTIC EQUIPMENT TO TROUBLE SHOOT THE PATS SECURILOCK SYSTEM. SOUND LIKE PROBLEM COULD BE KEY CODE.KEY COULD BE WORN DAMAGED.FAULT IN ECM OR IMMOBILIZER CONTROL MODULE. BEWARE WHEN THEFT SECURITY MODE TURN ON SOON OR LATER CAR WILL NOT CRANK OR ENGINE CUT OFF WHILE DRIVING.THEFT SECURITY SYSTEM WILL SHUT DOWN FUEL PUMP.

Okay,heres the problem the battery is low and the alarm is going crazy without enough voltage but something to try is to connect the jumper cables and have all doors shut. leave the cables on for a few minutes, if the alarm starts to go off as soon as you connect cables then dont wait, take the key and go to drivers door and lock and unlook the door lock with the key this will shut down alarm system and then try to start the car, it should start up and let run for a while, also the computer may have lost its idle memory so this means you will need to hold the gas pedal a little to keep it running, after a minute you can slowly release it and let it idle, you will know if this needs to be done when you start the car and it dies out, you will then strat it and hold gas, just a little bit enough to keep the engine running, hope this helps, have a good day.

I have had this problem on two separate honda accords. Can you keep the engine running if you turn the key to start and then let the key come back just a little? I have had two faulty switches. I could keep the engine running but I had to hold the key between run and start position.

is the car deiseling out .. what i mean is sputtering when you turn the key off and just not shutting down or is it running like you never even removed the key??? If it is chugging when you turn it off and not shutting down it sounds like the timing belt... hope this helps

If it's carbureted, you have an idle speed screw that can be adjusted. Here's a picture.

the computer is turning the car off and it is a thief device
you have a computer chip in your key and the computer reeds the code on it and will keep the car running. most likely your key
or chip needs to be replaced