2001 S60 2.4T, 88k miles. Until today, once warmed up, the temperature gauge sat dead center of the dial's range all the time. Today, it is flutuating from dead center up to just before the red block at the high end.

No engine lights or codes.

Waterpump was changed at 80k along with the timing belt service. Coolant was all replaced at the same time and is full. Radiator was replaced at 45k.

The engine cooling fan works, in fact, it is running like crazy it seems to keep things cool, and there is no reason as its 60 degrees outside.

I'm thinking the thermostat may be failing. What else should I be looking for?

 

Was the thermostat replaced with the water pump?

 

The thermostat was not replaced with the waterpump. We have had the vehicle since 42k miles. I do not know if it was ever replaced before we bought it.

 

Replaced the thermostat and, after two cross-town trips plus one freeway speed run the temp is again stable, and the engine cooling fan is back to normal. It seems my guess was correct. But this then leads to the question of which thermostat and which temp? I went with a Stant thermostat (less than $10) at the regular 190C temp because it is what my local parts store had.

Should I order a Volvo part instead? Is there any reason to go with the lower temp thermostat? Thoughts, theories and experiences welcome!

 

As a general rule, the modern automobile engine requires a higher temperature than its older counterparts. This is due to the closer tolerances. In fact, too low of a temperature can actually cause engine damage. Most engines today will operate at temperatures around 195° - 210°. The 190° should work just fine.

As a maintenance practice I flush and replace the coolant every five years to remove contaminants and renew the lubricating properties of the coolant.