I replaced my fuel pump a while back. Engine wasn't getting fuel. That fixed it.

While doing so the rollover valve broke. I ordered a new one but got one for a different truck. Seemed to fit okay and figured they all do basically the same thing.

Well I've had a problem ever seen of my tank building up a vacuum. Sometimes I can't start the truck. My solution was just to not close the gas cap all the way. I didn't fully understand the issue.

I replaced the carbon canister, and while doing so I discovered a lot of carbon build up. Couldn't blow through any of the hoses. I'm dumb- this has been my issue the entire time. Probably didn't even need to replace fuel pump.

Took an air compressor to them after Disconnecting. Had to replace a few.. all is freed up now.

I think I'm good. The one thing I don't understand though. There are two hoses coming from the carbon canister. The one going from rollover valve through canister to the little purge valve up front.

The other hose seems to go to nothing. Just a ball. Looks like a lever or something. Is that supposed to be that way?

 

Hope that picture is clear. Thats where the larger hose from my carbon canister terminates.

 

From what I can tell (I have a newer truck with the leak detection pump) that's correct. The rollover valve not only closes to prevent fuel leaking out in a rollover but also vents the tank in both directions. Both directions means when there is vapor building up it goes into the charcoal canister, and if there is a vacuum from temperature drop or fuel removal it allows air to be drawn in. That air should come from the tube pictured and the check ball should only allow air in and not vapor out.

 

Well that puts my mind at ease about it. A previous owner did a lot of questionable things to this truck so had to make sure he didn't just cork off a hose or something.

That makes a lot of sense. Since clearing the hose I no longer need to help my tank breath.

Just because I'm curious- Do you think a clog in the hose/ a break in the line would effect mpg and the accuracy of the fuel guage?

a break in the line leaking vapor/ a clogged hose and build up of pressure not letting the fuel guage level get a good read. Because I have been getting crap mpg and my fuel guage seems to drop to e when I got to a quarter tank

 

Need to add something to my previous response for completeness: The third and most important case where the tube in the engine compartment allows air into the EVAP system is when the purge valve opens and allows engine vacuum to draw vapors out of the charcoal canister.

No, I don't think issues with the EVAP lines cause bad MPG or an inaccurate fuel gauge. I believe the MPG is related to your other question about the O2 sensor, stay tuned...
The fuel gauge is a very simple analog design in the 97. Did you change just the fuel pump or the whole assembly ? Was that a problem before changing the pump ?