2007 Sierra WT 4.8 - Delayed start after Fuel Pump Replacement

[swampcritterr]

Hello All !

 

I recently had an intermittent crank-no-start issue on my truck (2007 GMC Sierra WT 4.8l). 

- I hooked up my little code reader and noticed that there was 0 fuel pressure during these times, Even if I cycled the key the pump wouldn't kick on then after a while it would fire up like normal.  I left the code reader plugged up and rode around when the truck did start and the fuel pressure was perfect (~60 psig on prime then 44-46psig while running/driving).

- I checked all fuses and the fuel pump relay, all tested good.

- I pulled the Fuel Pump Control Module and jumped across to turn the fuel pump on and found no issue, the pump responded every time I jumped the terminals and developed full pressure.

- Based on this I replaced the FPCM and had the truck towed to my local GMC Dealership to program the module.

- The technician said it already had the correct program and it wouldn't let him reprogram it but the truck still wouldn't start. 

- I asked them to run a diagnostic and they determined the fuel pump was bad.....( I had replaced the fuel pump ~ 2 years ago but used an aftermarket brand)  So I consented to replacing the pump with an AC Delco just so I wouldn't have to worry about it.

- So here I am $1,500 later with a new FPCM and Pump and the truck still has a delayed start any time it sits for more than 6 hrs or so. (I attached a video of it starting after sitting all night)

- I bought a mechanical fuel pressure gauge and hooked it to the rail:

      - After bleeding the pressure to 0 psig in the rail when the key is turned on (KOEO) the pump only develops ~ 15 psig and experiences a delayed start, then pressure jumps to 60-65psig and tapers off to 40-45psig.

      - After shutting the truck off the pressure rises slightly then falls about 10 psig over 4 hours (I haven't had a chance to leave it hooked and off longer than that yet).

      - *** After the first delayed start it starts like normal, just a few revolutions and fires right up like it had for the last 12 years. There are no performance issues, it never feels like it is starving for fuel and there are no Warning lights or error codes.  It does stumble and run a little rough every now and then but it has been doing that for a while now***

- I brought it back in and the technician couldn't reproduce the delayed start.  

 

Any ideas ?  Thanks in advance