I am trying to understand how a vacuum imploded my fuel tank twice. When driving up the road under normal circumstances, how is air supposed to get into the tank to replace the fuel used? I have a 1996 LS 4-DR 2-wheel drive with California evaporative emissions systems.

1) Through a one-way (outside to inside) vent in the gas cap?
2) Through the vent valve on the top front of the tank. I have heard this called a roll-over valve, so I assume it is not functioning except under those circumstances. However, mine is attached to the evap purge canister.
3) Through the vent valve for the evaporative purge canister, which is normally open.
4) Some other way. Please let me know.

 

If you 96 is the same as my 2000 then the vent line runs up to the filler neck. If it is the same I'd change out the gas cap. They are much cheaper than a new tank.

 

Are you saying that on your 2000 the gas cap has a vent from the outside to the inside of the normally closed fuel system and that this is the route taken by air that is replacing burned fuel? If so, can this vent be cleaned?

 

I don't know if it can be cleaned but a new cap should cost less than $10.

 

This detailed article "EVAP Evaporative Emission Control System" at Evaporative Emission Control System says that "On older EVAP systems, the tank is vented by a spring-loaded valve inside the gas cap. On newer vehicles, it is vented through the EVAP canister." Later describing the system components it says "Gas cap, which usually contains some type of pressure/vacuum relief valve for venting on older vehicles (pre-OBD II), but is sealed completely (no vents) on newer vehicles (1996 & newer)."

So for my 1996 and most other OBDII (1996 & newer) vehicles, air should not be entering the tank to replace burned fuel through the gas cap, and getting a new gas cap would not solve the problem of vacuum creation in the tank.

I have already confirmed that the hose to the vapor canister vent valve is not plugged and the vent valve was open when I tested it with the vehicle off.

 

Have you traced all your vaccumm lines? Don't over think it yet, I'd try a new gas cap. Worse case you're out $10.

 

Some research on my part has confirmed what I suspected for my 1996 (OBDII) Blazer LS, California emissions gas cap. That is, under normal operating conditions when the emissions system is not malfunctioning and there has not been an accident, nothing whatsoever is meant to pass either into or out of a properly screwed on and sealed gas cap. All normal operating pressure and vacuum relief goes through the evap vent valve up in front of the spare tire. That includes the vacuum generated by pumping gas from the tank to run the vehicle. This understanding came from the Stant #10836 replacement gas cap sales literature for my vehicle. That cap claims to have all the original factory features. It says:

"The cap prevents fuel vapors from exiting the fuel system under normal operating conditions. The cap allows an escape path for pressure in the event of an impact situation where the fuel tank has collapsed. It also permits pressure to escape if the vapor management system malfunctions. The cap allows an alternative path for air to flow into the tank if there is an obstruction in the vapor recovery or vapor management systems. It also permits the fuel system to normalize air pressure during natural vacuum situations." Does that mean driving up a mountain? I'm not sure what that last comment means, but it is certain that the cap is not meant to relieve the amount of vacuum generated inside the tank by the ECM to test for leaks in the emissions system under normal circumstances.

So the fuel tank cap for this vehicle is designed to release only the higher pressures and vacuums resulting from abnormal conditions such as an accident or malfunction of the emissions system, and nothing passes through the cap under normal operating conditions.