OK, this has been going on for several months so I have lots of clues but no answer yet.
Here's what I've got:.
When I turn the ignition on (not start) I can hear the fuel pump humming.
This humming sounds very normal. It's actually hard to hear unless it's quiet and you are listening for it.
What I have found is that the truck will not start as long as the pump is humming.
The humming normally only lasts a second or so and the truck starts right up.
But if I try to start it while it is humming, it will crank but not fire.
Sometimes lately, the humming goes on and on. I had to bum a ride home the other day because it seemed like it was just not going to ever start.
My mechanic changed the pump relay, just because it was easy and he wanted to eliminate that possibility.
I read that the ECM often has trouble with the capacitors, so I pulled it out and, sure enough, three capacitors had leaked.
I replaced them, but there was absolutely no change in symptoms.

It does not matter which tank/pump is on.

So now, when the problem is tolerable, I turn the key to "on" and listen to the pump. I just sit there - no jiggling keys, flipping switches or anything.
It will hum for a while, then the humming just stops. At that point I can turn the key to "start" and it will fire right up.
Like I said, though, lately it will keep humming for minutes on end with no luck.

I have to ask what exactly the humming is, and what exactly happens when the humming stops.
At first I just figured the humming was the natural sound of the pump running, but I question that since the engine will not start while the pump is humming, it starts when it is not humming.

Is the humming the sound of the pump trying to prime itself?
Does the humming just quiet down when it gets primed?
It seems hard to believe that both pumps have the same exact, weird problem - unless the problem is downstream.
Does the pump turn off when it hits a set pressure?
Normally, does the fuel pump run constantly or does it turn on and off, just supplying fuel to the high pressure pump as needed?

I'm at a loss here. (The only theory I have that fits all the clues is that she is mad at me for putting a "For Sale" sign on her.)

Thanks for reading all this!
And thanks in advance for any help you can give me!!

 

Read this page & the NEXT:

(click this text)

 

Thanks!
I did pull the ECM and replace three caps, but there was no change.
My next step will be to buy a rebuilt ECM (I've seen some as low as $100), unless someone has a better idea.

I would really like to understand better what the hum is. It sounds like a normal electric pump noise, but the engine will only start when it stops humming.

 

Steve83, I just re-read your post. You mention that a faulty ECM could cause "continuous fuel pump relay triggering".
What would be the symptoms of that? If the relay is being continuously triggered, would you hear a hum from the pump area?
What exactly does the relay do? Does it simply turn the pump on and off?
Thanks again! (I hope you have time to reply again.)

 

The symptom is that the fuel pump remains triggered continuously. Yes, the pump hums/buzzes/whines when it's running. Yes, a relay is just an electrically-operated switch.

 

So you can see my confusion.
Why does the engine NOT start when the pump is running (buzzing) but it WILL start when the pump STOPS running (buzzing).
I just crawled under there (and I AM too old for that!) and I waited until the buzzing stopped.
I was thinking that maybe the buzz I was hearing was an unnaturally loud buzz and that when it went away the pump might have a quieter buzz.
But when the buzzing stopped, I could not hear any noise at all from the pump. And yet at that point the engine will start.

 

When you read what I linked, you'll understand enough. The physics & electronic engineering behind it is too complex to explain - just accept that what's described is as deep as you need to go, and that many people like you fix it themselves at home for ~$30 including the tools.

 

OK, as I mentioned, I already pulled the ECM out and I replaced some of the capacitors.
That didn't seem to have any effect, so I just now ordered a rebuilt one.
I'll post an update once I install it.
Thanks to Steve83 for helping me along.