I need some help with some weird problem. This morning, not even a mile away from home my wife's 2000 s40 started revving up. she took her foot off the accelerator and it continued to accelerate. Then the temp gauge went all the way to the top then immediately came back down. This has happened once before while on the highway. Her temp gauge went up to hot so she pulled over and we had it towed to our volvo mechanic. He said that even though the gauge rose to full hot, he could not get the car to over heat. He replaced the Temp controller in the summer of 07 and 08. why does this keep happening? is it the temp controller again? Thermostat was replaced last summer. Cost us $150 last time with labor for the controller. Also the check engine light comes on and off at will and mechanic can not get it to stop coming on after reset. Please help!!!

 

I will bet the coolant temp sensor is bad.

 

I just went out and drove it around to see if i could get it to do it for me with no luck. i did feel a little rev from idle to 1500 rpm when i was parked. Can she drive it safely for now?

 

I would not try driving it to far because I have seen then rev and stay at like 2,000 RPM and it is a bear to stop it because the engine is not creating much vacuum for the brakes.

If she drives it make sure she does it carefully!!

 

Is it typical for the coolant temp sensors to go bad every year? Ill pick up a new one at the dealer and replace. In the bottom of the thermostat housing right?

 

yes. and i also agree that its the temp sensor...

 

ok, i will replace. should i replace the thermostat even though i replaced last summer?

 

I would see what it looks like when you get in there.

 

My car did the same thing and just quit starting a couple days after. It is your coolant sensor. It is a really easy fix and only cost about $65 at autozone. It is located right there on your thermostat