As soon as I star my s40 the idle races up to 4,000. Sometimes it jumps up to 4000 then drops down to normal, and I let it idle but if I try to restart it, it just cranks, doesn't start. I've checks for vacuum leaks, cleaned out the aic, the throttle cable isn't stuck.
Any ideas? Thinking maybe FPR?

 

I would remove the air hose to the throttle body and look at the throttle butterfly to make sure there isn't dirt or gunk holding it open. (Or a screw stuck in there! Long time ago on another car, sorry) The FPR shouldn't cause a high idle. I think you had the right idea looking for an air leak. It takes quite a bit of air to support 4k rpm. You can dump all the fuel you want in there but without air it's not gonna work.

Also check the exhaust manifold when it idles that high. Is it cherry red? That's a sign of a very lean motor from too much air.