Just bought an s70 and a few days later it started to overheat. On the way to work, I noticed kind of late that it was running close to the red. I immediately pulled over, let it cool off. The coolant was low so I threw some in. Drove fine afterwards.

The drive home, it ran slightly hotter than normal, but I made it home without the gauge rising a notch above normal. Not a long drive.

I flushed the coolant, it ran fine and cool. During inspection the heater core failed but it didn't overheat. I bypassed the heater core until a new one comes in.

Replaced the thermostat, although the old one didn't look bad. The only thing I can think of now is the water pump, although the previous owner claims to have changed it not long ago.

 

usually water pumps fail by leaking or spinning on their shafts so it doesn't sound like that's the problem. Changing out the thermostat is a good start. I'd consider swapping out the engine coolant temp sensor as well as that could be giving you an erroneous gauge reading (just had to do this on my S40 - gauge would creep up but no other symptoms). Also consider replacing the coolant overflow pressure cap and check the container for leaks. If you're still losing coolant with the heater bypassed, time to start figuring out where its going - ie look for leaks around the engine/radiator, or it could be in the turbo or head gasket (you're probably right about the heater core as the cause).

 

I am pretty sure the sensor is good and the reading is accurate. I will read the exact temperature tomorrow and find out for sure.

The pressure cap is new on the overflow, and it is not leaking. I have checked everywhere for leaks and there appears to be none.

The car is non-turbo, by the way.

I am beginning to suspect a bad headgasket.

 

does the upper radiator hose feel unusually firm to you? that can happen if some pressure from the cylinders blow by the head gasket into the cooling system.

 

It was actually quite soft even at operating temperature. Felt like coolant was flowing through it quite nicely, as well.

 

If you are not sure, I'd start with a new temp sensor. Earlier this year I replaced the ECT sensor on my S40 as my temp gauge would creep up to just under the red during normal driving and there were no other symptoms (ie no boil over, no hard hoses, no odd smells from a hotter than normal engine, no white puff on start up etc). Fixed the problem for me.

 

Any good source for an OEM sensor?

 

I shop fcpeuro.com, eeuroparts.com and tascaparts.com FCP offers both OEM and after market, EEuro has great prices and Tasca is mail order from a Volvo dealer and has the best prices on Volvo brand parts I've found yet.