My 2004.5 Volvo S40 2.4i overheated one night, and after getting a wrecker to take it home I figured it was the thermostat not letting coolant into the engine and all of the coolant just boiling away, due to air bubbles in my expasion tank. I replaced the thermostat and noticed 1. a slight dripping of coolant off the bottom of my engine block on the driver side (only when the car is running and at around 100 celcius) and 2. A slight boggle down in RPMs after revving. It adjusted the RPMs after they dropped to around 600 back up to the normal 800 range, so I figured it was a dirty MAF and throttle body. After cleaning those up it helped a bit but the leak was still very apparent. I decided to take the thermostat off and see if it was functioning in a boiling pot of water, and noticed the thermostat is missing a little relief valve looking thing inside it, compared to the old one. As well as having different numbers inscribed on them. How embarrassing I think I ordered the wrong one. I've tried to trace the leak but can't find the source. Is it a head gasket? Do I need to do a compression test? Drain the radiator? I heard they can get full of gunk sometimes. The expansion tank has fluid in it, and the car goes up to around 100 celcius and stays around there, but I drove it around my neighborhood and it hit 120 celcius at one point. Is the engine not getting coolant? The fan also takes a while to turn on. The car has 126xxx miles and I don't have any previous maintenance history besides finding stripped bolts on my thermostat housing. I'm committed to fixing it myself but I'm lost on where I should start. Any information will be extremely appreciated and I will answer any questions as fast as I can! Please help! I can take pictures of anything as needed. Thanks in advance!

 

did you consider the water pump? Is there any dripping under the timing belt cover? Does the heat come on in the cabin?

head gasket failures usually present several ways - coolant consumption into the cylinders producing a white/foggy exhaust - possibly with a sweet smell, oil in the coolant or coolant in the oil (any foaming on the dipstick or fill cap?). You can test for HCs in the coolant ($40 kit) - to see if exhaust gas is leaking in and you can do a compression test (typically to look for low compression in adjacent cylinders. Sometimes this will also put pressure into the coolant system to the point the upper hose gets rock hard.

As to your problem, what did the old thermostat look like? Do the radiator hoses get hot as the engine warms up? How long does it take before it overheats? does the heat come on in the cabin and if you put it on full does it control the engine temp?