Had two issues. fuel gauge was reading erratically and a had code for small evac leak. decided to drop the tank and take a look at things. inspected all the evac lines etc and found no issues. fuel pump looked a little worn on the contacts so i replaced it. put everything back together and the code disappeared.

Fuel gauge still erratic. so I crawled back under the car and low and behold the gas tank was sucked up like it was under major vacuum.so much so that it was physically interfering with the pump arm movement. gauge would go from full to 1/2 within 10 miles. the tank returns back to normal and goes back to full if you remove the gas cap.

so - i replaced the gas cap. still does it.
so- i replaced the vapor canister. still does it.
so - i replaced the purge valve. still does it.

pull the tank again and check all lines with compressed air. no obstructions found.lines appear good.you can blow air through them into the tank.

put it back together and it still acts up. tank sucks up, gauge drops down. returns back to normal if you unscrew the cap.

I am absolutely dumb founded at this point.

 

if the tank collapse started happening after you dropped the tank...you may have hooked up the pump or fuel lines wrong....happened to a friend of mine with his old ford.

 

all the lines are right. that was the first thing I checked.

I did figure out the problem. The Vapor Leak Detection Pump is what controls the fresh air inlet on this thing. Why they named it that I have no idea cause all it is is an open and close valve that lets the tank breathe. It had failed shut and therefore the tank was being sucked on by the purge valve. its located inside the rear quarter panel under neath the fill tube.

5 minute job and a 40 dollar part.

 

I looked real quick last night and couldn't find the valve, as apparently they don't call it that. Good info for others!