So, this morning before I left on a work trip I decided to change my downloader on my personal truck. In order to do this I switched off my pusher pump for the duration of the download. I went into the house, came back out a little later and unplugged the downloader. Started the truck to let it warm up a bit and after maybe a minute I realized that a certain buzzing noise was missing. I looked at my commander and it read 0 psi fuel pressure. I shut the truck off and started thinking... how was it running in the first place with no fuel pressure? Was the cp3 just drawing enough fuel in by vacuum?
I do still have the factory in tank pump, so it should have been running. But then why did the commander read 0 psi?
My work truck's commander does weird things with fuel pressure too. When going down the road everything is great, but when idling the pressure will bounce all over the place from 0-8 psi. A lot of times it will read 0 or 1 psi at idle for a minute or more.

 

The CP3 will draw fuel through the in-tank lift pump even if the LP isn't running, although it's not good for it.

IIRC, the Duramax CP3 isn't fed by an LP at all...

If the gauge is mechanical, try throttling the pressure line if you don't have an isolater (diaphragm).

 

Yeah, I had read that the duramax doesn't use a LP. If it doesn't need a LP then why does the Cummins? So long as the pump is pulling fuel in it should stay cool, right?

 

Oh, and the commander uses a sending unit mounted to the cp3, so no isolator since it is electrical.