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Had a problem this week I'm still trying to solve. Started losing power while pulling my boat to a local reservoir (97 Suburban 5.7 liter). Got past a grade I was pulling and it seemed to be fine. On the way home, had to pull another grade, problem returned but much worse. Had to stop the rig. Disconnected the boat to see if the Suburban could get me home without it. Heard some noise in the general area of the gas tank, hard to describe but not the fuel pump whine I heard in the past. On a whim, removed the gas cap (I had actually heard the gas tank making a 'thunk' sound while having the problems, and it relieved a bunch of pressure. Low and behold, it ran great with the cap off. Hooked back up to the boat and made it all the way home with no problems. Went to buy a new gas cap today, put it on, started having the same problem fairly quickly. Removed the cap, it went away. I checked the last few inches of the vent tube (from the filler neck to the rubber hose coming from the tank), no obstruction. For now, I am just leaving the cap just barely turned in, but I know this is not a good long-term solution. Any ideas? Anyone else have this problem? I really don't want to drop the tank ... I have had to do it twice before to change fuel pumps on this rig.
 

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You should have some rubber lines running from the top of the fuel tank to the charcoal canister and the TBI, they run along the frame rails, fuel tank venting actually goes through the charcoal canister then to the atmostphere, check to make sure none of these lines are collapsed or obstructed then check to make sure the charcoal canister is properly venting.
I have seen charcoal canisters get saturated with fuel and stop venting properly.
 

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You relieved a vacuum, not pressure. Static pressure in the tank would only help the pump feed the engine.

The gas cap is sealed, the venting is done through the line Tim mentioned.

There are three lines to the tank. 3/8, 5/16, and 1/4 inch. 3/8 is the supply line. Fuel is pumped through this line from the tank to the fuel metering body. (No TBI on a 97 Vortec). The 5/16 returns excess fuel from the metering body back to the tank. The 1/4 line is the vent line. All three lines come up the drivers side frame rail. The fuel lines go up over the trans to the engine. The vent line continues into the engine compartment along with the rear brake line. The vent line goes to vapor canister mounted near the radiator on the drivers side. If you remove the vent line from the canister, you should be able to blow through the line to the tank. Take the gas cap off first. If that line is blocked, you've found where the problem is. If not, the problem is in the canister or purge circuit.

The canister captures the vapors, which are drawn into the engine through the purge valve on the manifold. The vapors should not be escaping to atmosphere.
 

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Did a little experimenting with my own charcoal canister this past weekend, 96 Tahoe. There are three connections. Tank, Purge, Cleaner. Pretty obvious where they go. In my case, the "Cleaner" port just has a cap over it to prevent debris from getting in there. There are no check valves or restrictors. I could pull vacuum or push pressure through all three ports no problem.
 

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I’m also having this problem with my 2000 GMC Sierra SL c2500 5.7. It creates a vacuum and collapses the fuel tank. Im on my third fuel tank but didn’t realize at first a vacuum was crushing it. Any ideas? It seems the more I read, the more possibilities that could cause it but its getting expensive with the tanks and pumps. Service engine light, loses power or stalls, abs activating, and reading fuel rich on both banks. Mechanic shop is stumped. I did have a heater core hose corrode and had to be chiseled out and retapped and replaced aluminum fitting. I cannot fond any damage to other hoses or fittings. Maybe manifold gaskets?
 
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