My 2002 XC70 (113,000 miles) loses coolant when driven at highway speeds (55mph +) but the loss is not noticeable during city driving. After an hour on the highway the reservoir will empty and an emergency message will appear. It has been in the shop for pressure testing and close inspection twice and the reservoir cap has been replaced, but with no change. Any suggestions?

If you have the original water pump, it may be beginning to leak. Do you see any cooling fluid dripping onto the ground behind the right front wheel when you stop the car after highway driving? If so, it probably is the water pump.

If you had the timing belt changed on schedule, they should have done the water pump,too.

Could you describe this in more detail? Does the temperature gauge go up too high at any time (city or highway)? Does the coolant appear to be lost as soon as you stop after highway driving or does it seem to be gone the next day? It seems hard to imagine you would not see or smell some coolant or find some streaks if a half-full bottle of coolant disappeared in just an hour.

I wonder if the radiator may be obstructed - does it get almost as hot as the hoses or the block? I think if the circulation is bad the additional heat generated on sustained driving might stay in the block. The car might not overheat much due to air cooling as you move. But a joint or clamp might leak under the high pressure that would exist mainly then (and when cold could later let in air allowing an air pocket to form).

It suggests that there's a leak with a lot of resistance (to flow) -- after all, it mainly occurs when the highway speed results in faster rpm, thus higher water pump circulation pressure, thus greater pressure.

I'm thinking maybe a head gasket??? I'm not sure about this, but I'd give it some thought and keep my eyes open to some signs ....

Any oily deposits in the coolant tank reservoir? Any deposits around hoses, or sprayed around the engine compartment walls?

Good luck.