Low fuel rail pressure with aux tank

[john.firth.161]

I have a 2006 Silverado 3500 duramax with 50 gal aux tank in my bed. I am having issues when aux get below half tank I get Low Fuel pressure code and engine stalls. I can clear the code and switch to main tank and pump fuel at filter housing and runs fine.
Do you think if I add lift pump would help solve this issue. My truck has 174k and use to pull my 40ft fifth wheel. Everything is stock. I have electric switch in the cab to switch from main to aux. any suggestions would be appreciated


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[2004LB7]

how do you have it plumbed in?

 

[john.firth.161]

It was installed before I purchased. It look like main tank and aux are plumbed to the electric switch. I have not had any issues up to about 1 year ago when the issue happened after 40 gals used. Now it is just over half tank. No problem with main tank.


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[darkness]

Is the vent clogged up?

 

[john.firth.161]

Maybe stupid question but where would I find the vent.


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[darkness]

Well it should be somewhere at the top of the tank like a shrouded port or it could be the cap. A pic would help.