any ideas
 

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??

RE: any ideas
 

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.

RE: any ideas
 

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

RE: any ideas
 

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