can't keep 2000 S40 started

[scooter1972]

our S40 won't dtay started unless you are on the accelarator. It will not
run idle, and ran perfect prior to this problem.

 

[Big Dick]

Probably idle motor needs to be replaced.
It is called the "Isc valve idling" and a gasket for the valve.
It is mounted on the front of the throttle valve housing
towards the radiator area.

BD