Cranks but no start-fuel pump not coming on

So I bought a 2000 Expy to clean up and make a daily driver and it's giving me some serious headaches.
It's a 2000 EB 4wd w/ the 5.4, just for reference.

Occasionally, but not always, it will crank but not start. It seems to mostly do it when it's sat for a long period but it's hard to say for sure if that's a contributing factor.
I've been all over this thing and have narrowed it down a bit.
-The fuel pump works fine.
-Fuel pump relay is good, as is the fuse
-There are no stored or pending trouble codes
-"Theft" light blinks normally w/ key off (though faintly) and at key on it comes on solid for a few seconds and then goes off

Checking the fuel pump relay has revealed some oddities though.
When its acting up (no start) the PCM is not grounding the relay. There is power to the 2 legs that get power (30 for the high current and 85 for low current switch) however, the PCM does not ground the relay (86) and thus the truck is dead in the water.
If I remove the relay and add a jumper between 30 and 87 the pump comes on and it starts/runs just fine.

This thing is really getting on my nerves as I'm trying to get it road worthy but I need to be able to trust that it'll start every time.
I've kinda narrowed it down to some kind of issue with the PATS system or possibly an intermittent PCM failure.

Any ideas anyone???

Try replacing the fuel pump relay. Mine went South about a year ago. You can swap one next to it temporarily. My vehicle is same as yours. Try it, it's free.

Sent from my N9131 using Tapatalk

Even though I knew 100%, without a doubt, that the relay was good (bench tested it), swapping it out was one of the first tests I did.

The problem is that the PCM is not grounding the relay trigger to turn on the pump. I checked it at key-on (should be on for like 2sec to prime) and cranking and nothing, no ground.
I also checked the wiring from the PCM to the relay and it was fine.

If you thoroughly checked the fuses under the hood and under your dash regarding the fuel injectors, fuel pump and mass airflow sensor and all are not burnt then I would check your positive feed to your alternator, make sure that it's not corroded on the actual alternator and by the firewall.

Good luck!

Ordered a new PCM.
hopefully this will solve the issue. I'm not one to throw parts at a problem but I'm really running out of possibilities as I have checked and double checked everything and the only thing I can figure is an intermittent problem in the PCM itself. It's just that, in my experience, PCM failures are very uncommon. Still, the alternator has been replaced by the PO so he could have very easily suffered a dead battery and done a careless job jump-starting it and damaged the PCM. Who knows...

I'll try to post results and the eventual fix so that if anyone else suffers this problem there will at least be my experience in the search base.

The pcm also grounds the injectors in order to run so it may be a harness or connector issue on the fuel pump circuit.
You are also going to need the new pcm married to the instrument cluster by the dealer.

stamp11127 said:

The pcm also grounds the injectors in order to run so it may be a harness or connector issue on the fuel pump circuit.
You are also going to need the new pcm married to the instrument cluster by the dealer.

Click to expand...

The harness is fine. If I bypass the fuel pump relay (power the pump directly) the truck fires right up and runs fine. A continuity check shows the PCM is not grounding the fuel pump relay at first key turn (prime) or cranking. Everything else works as normal.
I figured the PCM might require dragging it to the stealership. I'll cross that road when the part shows up.

Are all your grounds good?

Mine just started doing this. I noticed if I pause for 2-sec, with key 'on', then start, it starts fine. I am guessing, I may have a leaking injector(causing a drop in fuel pressure, as it sits), a bad check ball in the fuel pump, a key going bad, something in the ignition or relay, not triggering/slow triggering of the fuel pump. In short, turn your key, slow, pause and start.

Reminds me of how I used to start up my '09 Dodge diesel pickup.