New member with a puzzle.
My 1999 5.0 Mountaineer would crank but would not start. After checking several things, I pulled the fuel pump relay and jumped the contacts in the fuse block. Turned the key and the car turned over and started. I then replaced the fuel pump relay with a new one and started the car several times successfully. Went back outside several hours later and the car would not start again. Did the jump thing again (car starts) and plugged in the relay and the car starts again. The non-start problem will reappear when the car sits for a couple of hours after jumpering and replacing the relay.
Anyone have any idea why the car will not start after sitting for a couple of hours?

Thank you
Klaus Cook

 

There is a "Fuel Pump Driver Module" (box) that gets the signal to operate the fuel pump. If it does not relay that signal on to the fuel pump ,no fuel is pumped to the engine.
Some are located under the rear of the vehicle and develop a corrosion problem and some are in the trunk. Power to the fuel pump should be confirmed when the no start condition is present. If power is absent go back to the driver module and make sure it is sending that signal to the fuel pump. Most of the time the driver module is the culprit.

 

Thanks for the quick reply.
Power to the fuel pump is present at the relay in the underhood fuse panel at all times. After jumpering the contacts of the fuel pump relay the car will start normally. If I replace the relay, the car will still start normally for about 1 -2 hours. Then it quits again. I can duplicate this cycle by jumpering the power contacts of the fuel pump, starting the engine and then replacing the relay.

 

This is what I would do,
In order to verify full power is getting to the driver module , you will need to have or use a voltmeter to prove this,. You may need to lift the relay panel to do this, but it is either measure it at the fuse panel or at the driver module. Have the test leads connected to the output terminal of the relay and let the vehicle set for a while. Then after a period of time turn on the voltmeter and attempt to start the vehicle. If you see 12 volts or more coming out of the relay on down to the driver module , the relay is doing it's job and is not the root cause of the problem.
This is what sometimes happens,
Poor connections or faulty modules will function temporarily once enough power is repeatedly applied and yet not function if there is the slightest loss of power (poor connection) or faulty component, The meter will show if that is the case. Playing with the relay is doing just that and is not verifying anything..