Dake

Got a doozy of a first post for you guys. I just bought my 2000 Suburban 2500 about a month ago. On the highway trip home we got about 18MPG which as far as I can tell is crazy for a 6.0. After a few trips down my gravel road it started having the typical fuel filling issue (won't accept fuel without the nozzle hanging partway out) and also the tank started to pull so much vacuum that it was cavitating. Collapsing like a water bottle. So obviously something is plugged somewhere in the evap system.

I crawl underneath and discover that the air hose for the rectangular vent valve is completely disconnected in such a way that it hasn't been connected in a very long time. I remove the valve and blow air through it, and a lot of dust comes out. I also remove the purge valve at the engine and do the same, also making sure that it actuated. All good there and no carbon pellets at either end.

At this point I'm getting too busy with other projects so I take it to my local Chevy dealer. They drop the carbon canister and diagnose it to be perfectly fine. Then they tell me there is supposed to be an electrical connection to the vent valve rectangle box and it's nowhere to be found. I never unplugged it when I took it off and to their eyes there are no witness marks that one has ever been plugged in to this box.

I thought I read somewhere that certain years of 2500 had a separate solenoid from the rectangular box. I also thought I read that in certain years this rectangular box is basically passive and only acting as a filter for incoming air and didn't actually have any electronics inside. In short, I have no idea what's going on. And the dealer doesn't either. They even contacted GM corporate who insisted there should be a wiring harness somewhere.

At this point our best guess is the harness is hiding somewhere above one of the tanks. The tanks are too full to smoke the system. The only other variable here is that there is an aftermarket gas cap. I actually replaced it with a second aftermarket gas cap and nothing changed. I can drive the Burb with no gas cap and get no check engine light, but it drives like crap. I'm starting to think the entire drive home where we got 18MPG we were actually running lean the whole time on account of this issue. But I have no idea, and the dealer has no idea, so here I am.

And here's a picture of the Burb. Soon to be very trusty I'm sure.