Our 2004 s40 has been having strange issues. It started acting up this winter. Once the car warms up the temperature gauge will swing all the way up, red temp light comes on, and it surges (accelerates by itself) then the gauge will drop all the way down, stops surging when this happens, and the car will then sometimes stall at a stop unless we slightly keep revving the engine, it goes back and forth between these. ALSO, after driving the car, no matter how short a drive, it then won't start again until it sits for approximately an hour. We've replaced the maf sensor, any ideas? Please help, we need this car to work to be able to live! Thank you everyone!

 

Start by testing/replacing the engine coolant temp sensor. Its located under the thermostat housing cover / upper radiator hose so plan on also replacing the thermostat at the same time since you'll need to pop off that cover to access the sensor. Worst part of the job is figuring out how to unclip the connector :-). Also use some PB Blaster to pre-lube the two torx bolts holding the cover on. You don't want to strip those bolts.

 

I agree temp surging up and down is normaly a faulty temp water sensor , if its just the thermostat it will run hot and overheat when its stuck closed , or run too cold when on a highway when stuck open , in your case the temp gauge shows high and while still driving drops to cold must be the sensor
If you dont have the correct tool to remove the torx bolts dont try ,
On older cars with no sensors when the thermostat failed in the close position it was normaly a rebuild!