LB7: Sending unit questions


'02 3500 w/Srvc Bed
Feb 8, 2020
Hey everyone. First post after many months of perusing the site for info. Lots of frustrated hours and every inch of fuel line replaced, but I have a few questions that I cant seem to pin down answers to. First off, what is the middle port of the sending unit in the fuel tank for? Is something supposed to be plugged in there? Secondly, is the fuel tank supposed to be air tight? I've had low rail pressure codes since I bought the truck off my old man, and he had them for almost two years. The truck didnt have the power it used to, but still did everything he needed it to. CP3 has been replaced (with a used pump)
Filter head has been rebuilt, all soft lines replaced, and an Edelbrock in line pump (part#17302) installed to give the CP3 a little help. I'm putting everything back together now after replacing all of the fuel lines from the tank to the hard lines next to the glow plug module with reinforced nitrile lines. Any advice would be appreciated.


Why do I keep doing this?
Staff member
Apr 19, 2008
Mid Michigan
Moved this over to the LB7 section.
IIRC, that second port is deadheaded. Who knows what GM had planned for it.
No, the tank isnt airtight. There should be a vent that runs up thru the fill neck.
What specifically was the low rail code? That could be leaking injectors, bad CP3, etc....

Welcome to DD. :)

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