also had the same problem - make sure you check that
vacuum line to shutoff valve is on and the extender hose
is still good (its right next to the oil filter towards front of
car) sometimes replacing the small extender hose will help
as need a tight connection for the vacuum to activate
the shut off valve and turn the car off. On the other hand
it just mite ''self heal'' as happened to mine
ps: u can always manually turn the car off by pushing the
lever on the throttle linkage with the word 'stop' written all
over it - guess mb engineers knew the valve would fail sooner
or later!