fuel shutoff valve problems 1984 190d
 

If i go for a ride any longer than 15 minutes than the fuel shutoff vale shuts off. So i thought is was the fuel shutoff valve but it works fine before that time. If i start it up and just drive down the road than when i turn the key to shut it off it shuts off. So that makes me think its not the fuel shutoff valveitself because if it were broken it wouldnt shutoff. i also tried taking the vaccuum off of it to see if it would still shutoff after a 15 minute ride and it did. So right now i have the valve held up with a spring and every time i want to shut the car off i have to pop the hood and push the vavle lever down with a screw driver to shut it off. other than that the cars runs fine. So im wonderin if there is some other mechanical thing connected to the valve that is shutting it off when the engine gets warm. Any help would be great.