I have an 97 850glt.with the warm weather it some times boils over but the temp guage does not go above 3 oclock.rad fan does come on.No oil or water mixing.Any ideas want could be wrong?I will replace the thermostat anything else I should check?

 

Bad head gasket. Read all the details under the picture.

Head Removal and Installation, Volvo 850 / S70 Turbo B5234T, B5254T, Head gasket replacement, valve replacement.

 

does it boil like this in the video below? if it does the same your head gasket is shot.

 

I think it may be the begaining stage of rspi's post nothing like slimflex2's post.

 

When my 850 had a bad HG, it never ran hot either. Pull the oil cap (or dipstick) after the car has cooled off a little bit and if you see some milkshake-looking residue on either, you know it's time to change that stuff. If you are more sophisticated you can test for combustion gases in your coolant. The video above is a dead give away tho.

 

I have heard of some stuff that you can put in the motor to seal those small leaks but I only put oil and coolant in my car so I didn't try it.

 

Nor would I recommend it!