My '90 535i is always hard to start when it has warmed up and been driven for a while and then parked. When I go to restart it a few hours later, it takes a few cranks to get it going and then it stumbles and I can smell fuel, as if it's getting too much fuel. I have purchased the inline check valve that is supposed to be placed just outside the fuel tank. Correct? Any ideas on this problem or how to properly install this check valve? Thanks!