my sons 97 blazer we replaced fuel pump we checked for leaky tank and tank was good all hoses were replaced connections were tight fuel gage worked again truck ran great with no leaks gas cap was misplaced so i bought new one. the next morning my son drove to work (about 12 mi) and said gas was leaking like crazy and the truck was shuddering while driving warm. i went picked up truck and drove it no leak dropped tank again replaced hose clamps (everything was tite) started with tank down no leaks let it idle it started to leak pretty good i went to gas cap and undid it and it blew out air then it stopped spitting gas drilled a hole in the gas cap to vent it but truck still shudders somtimes while driving its not leaking from conections where could it be coming from it looked like it was coming from some box on the frame in front of the tank and above differential it has some type of hoses going to it and a wire connecter im stumped dropped tank twice already where is this fuel coming from and why does it stop when i undo gas cap

 

There really is no way for the tank to become pressurized unless it is from heat somehow. Now a vacuum on the tank is typical given the emissions system configuration.

You really need to figure out exactly where it is leaking from as well as check out the emissions system. There should be a purge valve on the engine and a vent valve back near the tank. The EVAP canister should be mounted behind the driver side rear tire and lines will connect the tank, canister, purge & vent valves.

 

Is there a book or manual that shows the complete fuel system with a diagram and its flow and the tank pulled in air not blew out sorry .

 

I just replaced the fuel tank on my 96 Jimmy and after that was done, big leak up near the fuel filter. The designers routed the steel fuel lines on top of the frame which get layers of dirt on top and cannot be cleaned. After a while the fuel lines rust and there is nothing to do but cut out and splice in new line or replace the whole thing. Not an easy task. Mine had a tiny pin hole and only leaked when the ignition was on. At around 60psi, a good amount of fuel came out.

Then.....after all that was done, I got a check engine light. Forward on the tank is some sort of pressure releif valve connected to evap system. When I filled the tank the first time, the vacuum hose had come loose and gas was pissing out of there until I ran it down a little. Still can't reconnect the hose without dropping the tank again so am living with a check engine light and I don't fill the tank fully.