I have just purchased a 1990 Oldsmobile ciera for cheap knowing that the ignition needs to be replaced but i don't believe its the ignition if the drive for more than 15miles whenever i turn it off it wont start back up until 40 mins later

Hi. Remove and check fuel filter. Replace. They're cheap. You should be able to blow freely through the old one. If not, the fuel pump has probably gone bad from working itself to death pushing fuel through the clogged filter. The pump overheats and when you shut if off it locks up until the gasoline in the tank cools it back down. Then it will start up and get you home. There are a lot of other things it could be, but this seems to be to lions share of failures. Nobody changes the fuel filters as regular maintaince and they should. To be reasonably sure, before you first start it have a friend listen to the fuel tank fill with the cap off. Turn the key to on, not start, just on. He/she should hear a distinct buzzing for about 2 secs. from the filler. Start up and drive the required distance for failure. When you go to start back up, repeat the test. If there is no buzzing when you turn on the key(again, only to on) the pump is overheated. If it sounds the same, it is a different problem. You didn't say if the problem always occured. Sometimes, if the tank is full it will cool off quicker. On the fuel filter; a lot of times you will find a new one already on there. That's because someone already tried it. This is particularly the case if you've just purchased the car as you have. It can still be the pump. Do the old listen test. If it passes that test, it could be a ECU problem or a group of other things that don't fail that often.