My 1998 V90 Wagon has approx 186k Miles on it. I've been experiencing substantial loss of coolant and reservoir requires topping up every day or two after 50-100 miles driving. I Never smell the telltale antifreeze leak odour. My mechanic replaced water pump last week (it had 100k miles on it), and said it was leaking badly. I expected this would eliminate the problem i've been having, but coolant loss continues. Mechanic pressure tested system and could not locate any leaks. After driving it for a day and when engine is cold and i open reservoir cap to top up, the coolant system is still under pressure or vacuum (cannot be sure which), and coolant level in reservoir rises up, indicating (to me at least) that there is a gas (air or exhaust gas pocket) in the system which expands upon loss of system pressure (Boyles Law) and displaces the coolant up into the reservoir. I can only think of 2 possible causes for this but maybe some experts out there have other ideas.
Possibility #1 - Coolant loss into cylinders through engine head gasket leak when engine is hot, but not sufficient leak to be detected with pressure test when system is cold. Not sure how this would explain the displacement of fluid in the reservoir upon removal of the cap, unless when the engine is shut off after driving there is exhaust gas leakage into the coolant system. Negative tested for exhaust gas in coolant about a year ago.
Possibility #2 - Coolant loss elsewhere as from a hose connection only when engine is hot, and when car is parked and engine cools, air is drawn in through the leak location due to vacuum pressure created when coolant circuit fluid changes temperature and contracts.
If i sound technical i am (engineer), but i'm not an auto mechanic or expert on engines i know some of you are out there.

Should i retest for exhaust gas leakage in coolant? or what else could explain upward displacement of reservoir level upon loosening reservoir cap? I have the green coloured reservoir cap, in case your wondering.

Hope someone can help.

Hi- I'm having almost the exact same problem w/ my 98 S90 sedan. It's got about 60k miles on it. Had the water pump replaced last week and the coolant light started going on again every 2-3 days with coolant loss. I have to stop and replenish the reservoir with about 2-3 cups of water at a time. They also fixed my a/c last week & since then the passenger side floor is soaked with (?) water (?). I don't know if these things are related but if anyone can help, it would be much appreciated. Thanks.

I am not sure if your models have the same overflow tank as my 96 850 and 99 C70 but both had this problem and it was small cracks in the coolant tank. On the 850 it was very difficult to find because is was very slow but it was on the bottom where the hose attached. On the C70 it was larger but in the same place. Both cars were plus 150,000 miles. This is a very easy do it yourself fix. Just order a tank, and remove the old one, pull out the level sensor and reinstall.

owen - i'm going to try what you suggested and radio feedback when i have a chance. thanks!

i got a new coolant tank and cap installed last week and there isn't any difference to the problem. i've still a leak somewhere in the system.

also, the radiator is brand new (and the radiators are plastic, not metal).
i don't have a problem with topping up the fluid every two days; its a good responsibility and encourages me to look under the bonnet at other components.

oddly, even my mechanic couldn't identify the source of the leak.

Here's a possible answer.... The radiator for the S90 has plastic fittings that the top and bottom hoses clamp to. They tend to get brittle and eventually crack. I know, I just replaced a radiator on my S90. Check and see if you don't have a leak there.