I reckon I go through 1 - 2 coolant tanks a year. My 850 will be going along fine and then one day, out of the blue, it boils over. Thing is, the temp gauge doesn't suggested an overheated engine, it's still sitting in the mid-range. The gauge seems to work. In the morning, when it's cold, it's where it should be, but then, as the car warms up, it sits in the middle. It never goes very high.

If it was the thermostat, wouldn't the issue be more frequent? If it was a faulty gauge, why would it operate normally at lower temps? I had some hoses replaced, but I don't think all.

Any ideas?

 

what do you mean "go through 1-2 coolant tanks a year" ? are you saying you need to replace the overflow reservoir on a regular basis? When you replace the tank are you also replacing the pressure cap and using the correct model (caps are set to blow at a specific pressure ) Are you losing coolant and need to top off on a regular basis? Sometimes a wonky temp gauge points to a failing ECT sensor (which screws in right below the thermostat - so typically you'd replace both at the same time).

Also have you ever had the system pressure measured or felt the radiator hose to see if its getting rock hard due to high system pressure? Overheating can be caused by a wonky thermostat, failing water pump (check for leaks by removing the timing belt cover ), a clogged radiator core, head gasket issues or a low coolant level. If you've never replaced the thermostat I'd suggest a new one (go OEM if possible) and the ECT sensor and make sure you have a correct reservoir cap. Next is to have a shop measure the system pressure on a warm engine.

 

Yes, it'd boil over and my mechanic would tell me that the reservoir was cracked and so would replace it. It's doubtful that he replaced the cap, too. Generally, it doesn't lose coolant, not beyond what is normal, but then suddenly it'll one day boil over. I've never had it pressure tested. I spoke with my (new) mechanic today who asked me to check that the fan was kicking in. I did, and it isn't. I asked if this can be an infrequent thing, as my issue is infrequent, and he said that yes, it can be. I've got it booked in next week. Any thoughts on the fan? Does that fan itself fail, or are there sensors etc. that can fail?

Thanks for your help.