I recently purchased a '98 V70 AWD and noticed that the coolant tank was empty. Even after filling it, the level would still disappear gradually until the tank was empty again. It seemed to be coming from the cap, so I replaced the cap and the leak continued. Upon closer inspection, the coolant expansion tank itself had cracked around the top where the cap screws on, so I did a complete coolant flush, and replaced the tank. I've had the system pressure tested to 15psi, and there are no leaks from hoses or the rad. The fan relay has also been replaced, and the fan comes on after several minutes of idling or if the A/C is turned on.
Despite all of this, it seems that after a long trip of 600km or more, the coolant tank will drain itself overnight. I'm driving from east coast Canada to Los Angeles right now, and have just made it to Las Vegas (about 4800km so far). Every morning (after 1100km the day before) I would have to refill the coolant as the tank would be empty. Aside from this, the temperature would stay constantly at the 3 o'clock position. Today, about a mile from Vegas and 44 degrees celcius (the hottest temperatures experienced yet), the temperature moved into the red zone, and the lambda light turned on. I pulled the car off the highway immediately and idled for 15 minutes with the heat at full blast. Even so, after another mile or so the temperature would gradually move up.
Any ideas on what could be causing this coolant loss, and what I could do about it? What about the lamda light? I trust that despite the high temperatures here in the desert, this overheating is still not normal.