I probably already know the answer............
I just bought this car a few months ago, the previous owner had a lot of work done to it, including $2700 of engine work. Then the heater core gave out, and they were through putting money into it. I got it for $850, with 122,000 miles on it.
It has been using a little bit of water, a pint or so every few days. Then to top molded radiator hose burst right near the thermostat. I had no way to get another, so I cut the end off, and it seemed sound, so reattached it. I have since ordered a new hose. One interesting thing is that even when the water level light is on, the temp never went above 1/2 way.
So on Saturday, we took her out for a 75 mile drive. It ran great, but doing some around town driving, the level light came on, and then the oil light flashed a few times. Then engine then died, and would not restart.
I found the top hose split wide open, beyond redemption this time. I had a new one brought over, but it is the corrugated universal style. I filled the coolant, and it started right up and seemed run well. It did have a check engine light on, which it never had before.
After about 20 minutes on the highway, the coolant low light came on again, and 3 times the check engine light flashed and the engine ran rough for a few minutes. Once again the temp was 1/2 way on the gauge.
We pulled over after about 45 minutes total, since the temp seemed fine. I let it sit for awhile, the slowly opened the cap, it took quite awhile to bleed off. Once that was done, I filled it up with coolant, slowly. It took probably 1/2 gallon, but some of that had bubbled over while loosening the cap. I also assumed it "burped" out an air bubble.
The last hour it ran great the whole way home. The low level never came on, and the the temp was a little less than 1/2. The check eng light was still lit, of course.
The next morning, I decided to check the coolant level, when I opened the resevoir cap, quite a bit of pressure hissed out. The water was only down a little bit, less than a pint.
I drove it this afternoon, and it smoked white for a couple of minutes, then stopped and ran fine.
Head gasket, or cracked head? ARRRRGGJJJ
Thanks
John

 

I would expect bubbles to be prominent in the header tank while the engine was running if you had a gasket or head problem. The fact that you have to add water so frequently means it's leaking somewhere.Park over concrete or tarmac and see if there is a puddle forming overnight. If that much water was finding it's way into the sump your oil would be soup.These cars also do not have a air bleed point on the cooling system and if the water pump is not flooded they will not purge all the air but as you have been driving it a bit I don't think that fits your symptoms. They boil real quick when this happens. Find the leak!

 

Also as far as your check engine light is concerned you can usually extinguish those by disconnecting the battery for a few secs and then reattaching but doing so mean you have to reenter the radio security code. If you don't know it that's a PIA but sometimes it's written on the radio chassis. There are slots on either side of the facia plate to withdraw the radio. Don't go to Volvo dealer to get that light dealt to, they will take all your money if they get a chance. Another option is a trans shop, they have OBD gear and will probably do it for a 6 pack.

 

So, I drive it to work this morning. It had white smoke out the exhaust for 15 minutes until the engine was good and warm, then it stopped. It ran perfectly normal.
After shutdown, I looked back, and lots of white smoke out of the tailpipe, for about 30 seconds. Then it stopped.
I question the accuracy of the temp gauge, it has never gone above 1/2 way, even when very low on water and obviously overheating.
Thanks again
John

 

Question, how much pressure should the reservoir cap hold? It has as much pressure as a conventional radiator cap. Does it need replaced?

 

The reservoir cap should be holding system pressure. Excess is vented out through a hose on the top of the tank.This white smoke worries me a little.Is it smoke or steam? As far as the temp gauge reading is concerned the sender is near the top of the engine so if the coolant level is down below the height of the sender it is not in the water and will not give a true reading but the reservoir warning light should alert you to this. Sounds like your cap is doing what it should.