I have a 2000 S40, and I just had the weirdest problem.

I was driving in traffic when the rpms all of a sudden went up and stayed high. I drove a little longer trying to get the idle down, by tapping the gas, etc. They just stayed high and the car accelerated to where the rpms were. I really had to use the brakes as hard as could to stop entirely. Then I put the car in neutral and the rpms went way up since it had no load. I shut off the car then noticed the temperature went to the red. I tried to start it again but it wouldn't turn over. I had no warning or check engine lights at all.
I left the car for 1.5 hours, checked he fluids and coolant which were fine. Then I started the car (with no problem) then drove it all the way home (20 miles) without a problem. What could this be????????????
I don't want to drive the car when it does something this crazy. Help please??

 

Sounds like the thermostat may have gotten stuck closed temporarily. I would change that just to be safe. High temps will affect the engine performance.

 

Would the thermostat sticking actually cause the engine to stay at higher revs and not come down to idle??

 

This problem ended up being the temperature sending unit, underneath the thermostat, if this ever happens to anyone.