Gas tank building up excessive pressure, not venting properly.
 

Hello!
A buddy of mine has a 1993 Grand Marquis 4.6L V8 and something is up with the evap system as the tank is building up excessive pressure. When you loosen the gas cap you immediately hear pressure escaping from the tank and you can see a great deal of vapors escaping from the tank as well.

I suspected that either the canister purge valve solenoid or the charcoal canister were bad so we removed them. The tank still builds up pressure. I was able to blow compressed air through the canister and solenoid so I know neither of them are blocked.

I tried blowing air into the line that goes from the charcoal canister to the gas tank and it appears to be blocked. Now for the million dollar question....does the line that goes from the charcoal canister to the gas tank connect directly to the gas tank or is there some sort of check valve or sensor at the tank that the hose connects to?

The evap diagram on the hood shows that the line out of the charcoal canister goes to the gas tank. It does not show any sort of valve/sensor. My buddy has the Haynes manual but we know how worthless those are in situations like this. It does not show an extensive evap diagram.

I have googled my heart out trying to find the answer to this as well as an evap diagram but have had no luck. Any help on this would be greatly appreciated.

Thanks in advance!